c8h4/postgresql
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Update FAQ_DEV.

Browse source
This commit is contained in:
Bruce Momjian 2000-12-09 04:29:43 +00:00
parent 5eced96f2c
commit 5dd9fc724e
2 changed files with 548 additions and 503 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

180
doc/FAQ_DEV
View file

@ -27,6 +27,7 @@
11) What is configure all about?
12) How do I add a new port?
13) What is CommandCounterIncrement()?
13) Why don't we use threads in the backend?
_________________________________________________________________
1) What tools are available for developers?
@ -34,23 +35,22 @@
Aside from the User documentation mentioned in the regular FAQ, there
are several development tools available. First, all the files in the
/tools directory are designed for developers.
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directorie
s
ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static
find_typedef get a list of typedefs in the source code
make_ctags make vi 'tags' file in each directory
make_diff make *.orig and diffs of source
make_etags make emacs 'etags' files
make_keywords.README make comparison of our keywords and SQL'92
make_mkid make mkid ID files
mkldexport create AIX exports file
pgindent indents C source files
pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directories
ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static
find_typedef get a list of typedefs in the source code
make_ctags make vi 'tags' file in each directory
make_diff make *.orig and diffs of source
make_etags make emacs 'etags' files
make_keywords.README make comparison of our keywords and SQL'92
make_mkid make mkid ID files
mkldexport create AIX exports file
pgindent indents C source files
pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog
Let me note some of these. If you point your browser at the
file:/usr/local/src/pgsql/src/tools/backend/index.html directory, you
@ -71,9 +71,9 @@ s
support this via tags or etags files.
Third, you need to get id-utils from:
ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz
ftp://tug.org/gnu/id-utils-3.2d.tar.gz
ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz
ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz
ftp://tug.org/gnu/id-utils-3.2d.tar.gz
ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz
By running tools/make_mkid, an archive of source symbols can be
created that can be rapidly queried like grep or edited. Others prefer
@ -85,40 +85,39 @@ s
Our standard format is to indent each code level with one tab, where
each tab is four spaces. You will need to set your editor to display
tabs as four spaces:
vi in ~/.exrc:
set tabstop=4
set sw=4
more:
more -x4
less:
less -x4
emacs:
M-x set-variable tab-width
or
; Cmd to set tab stops &etc for working with PostgreSQL code
vi in ~/.exrc:
set tabstop=4
set sw=4
more:
more -x4
less:
less -x4
emacs:
M-x set-variable tab-width
or
; Cmd to set tab stops & indenting for working with PostgreSQL code
(c-add-style "pgsql"
'("bsd"
'("bsd"
(indent-tabs-mode . t)
(c-basic-offset . 4)
(tab-width . 4)
(c-offsets-alist .
(c-offsets-alist .
((case-label . +))))
t) ; t = set this mode on
and add this to your autoload list (modify file path in macro):
and add this to your autoload list (modify file path in macro):
(setq auto-mode-alist
(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-
c-mode)
auto-mode-alist))
or
/*
* Local variables:
* tab-width: 4
* c-indent-level: 4
* c-basic-offset: 4
* End:
*/
(setq auto-mode-alist
(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
auto-mode-alist))
or
/*
* Local variables:
* tab-width: 4
* c-indent-level: 4
* c-basic-offset: 4
* End:
*/
pgindent will the format code by specifying flags to your operating
system's utility indent.
@ -174,8 +173,7 @@ c-mode)
a typical code snipped that loops through a List containing Var
*'s and processes each one:
List *i, *list;
List *i, *list;
foreach(i, list)
{
@ -207,17 +205,15 @@ c-mode)
You can print nodes easily inside gdb. First, to disable output
truncation when you use the gdb print command:
(gdb) set print elements 0
(gdb) set print elements 0
Instead of printing values in gdb format, you can use the next two
commands to print out List, Node, and structure contents in a verbose
format that is easier to understand. List's are unrolled into nodes,
and nodes are printed in detail. The first prints in a short format,
and the second in a long format:
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
The output appears in the postmaster log file, or on your screen if
you are running a backend directly without a postmaster.
@ -292,11 +288,11 @@ c-mode)
tables in columns of type Name. Name is a fixed-length,
null-terminated type of NAMEDATALEN bytes. (The default value for
NAMEDATALEN is 32 bytes.)
typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length,
@ -311,8 +307,8 @@ c-mode)
9) How do I efficiently access information in tables from the backend code?
You first need to find the tuples(rows) you are interested in. There
are two ways. First, SearchSysCache() and related functions allow
you to query the system catalogs. This is the preferred way to access
are two ways. First, SearchSysCache() and related functions allow you
to query the system catalogs. This is the preferred way to access
system tables, because the first call to the cache loads the needed
rows, and future requests can return the results without accessing the
base table. The caches use system table indexes to look up tuples. A
@ -321,13 +317,14 @@ c-mode)
src/backend/utils/cache/lsyscache.c contains many column-specific
cache lookup functions.
The rows returned are cache-owned versions of the heap rows. Therefore,
you must not modify or delete the tuple returned by SearchSysCache().
What you *should* do is release it with ReleaseSysCache() when you are
done using it; this informs the cache that it can discard that tuple
if necessary. If you neglect to call ReleaseSysCache(), then the cache
entry will remain locked in the cache until end of transaction, which is
tolerable but not very desirable.
The rows returned are cache-owned versions of the heap rows.
Therefore, you must not modify or delete the tuple returned by
SearchSysCache(). What you should do is release it with
ReleaseSysCache() when you are done using it; this informs the cache
that it can discard that tuple if necessary. If you neglect to call
ReleaseSysCache(), then the cache entry will remain locked in the
cache until end of transaction, which is tolerable but not very
desirable.
If you can't use the system cache, you will need to retrieve the data
directly from the heap table, using the buffer cache that is shared by
@ -344,28 +341,26 @@ c-mode)
While scans automatically lock/unlock rows from the buffer cache, with
heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it
when completed.
Once you have the row, you can get data that is common to all tuples,
like t_self and t_oid, by merely accessing the HeapTuple structure
entries. If you need a table-specific column, you should take the
HeapTuple pointer, and use the GETSTRUCT() macro to access the
table-specific start of the tuple. You then cast the pointer as a
Form_pg_proc pointer if you are accessing the pg_proc table, or
Form_pg_type if you are accessing pg_type. You can then access the
columns by using a structure pointer:
((Form_pg_class) GETSTRUCT(tuple))->relnatts
Once you have the row, you can get data that is common
to all tuples, like t_self and t_oid, by merely accessing the
HeapTuple structure entries. If you need a table-specific column, you
should take the HeapTuple pointer, and use the GETSTRUCT() macro to
access the table-specific start of the tuple. You then cast the
pointer as a Form_pg_proc pointer if you are accessing the pg_proc
table, or Form_pg_type if you are accessing pg_type. You can then
access the columns by using a structure pointer:
((Form_pg_class) GETSTRUCT(tuple))->relnatts
You must not directly change live tuples in this way. The best way
is to use heap_modifytuple() and pass it your original tuple, and the
values you want changed. It returns a palloc'ed tuple, which you
pass to heap_replace(). You can delete tuples by passing the tuple's
t_self to heap_destroy(). You use t_self for heap_update() too.
Remember, tuples can be either system cache copies, which may go away
after you call ReleaseSysCache(), or read directly from disk buffers,
which go away when you heap_getnext(), heap_endscan, or ReleaseBuffer(),
in the heap_fetch() case. Or it may be a palloc'ed tuple, that you must
You must not directly change live tuples in this way. The best way is
to use heap_modifytuple() and pass it your original tuple, and the
values you want changed. It returns a palloc'ed tuple, which you pass
to heap_replace(). You can delete tuples by passing the tuple's t_self
to heap_destroy(). You use t_self for heap_update() too. Remember,
tuples can be either system cache copies, which may go away after you
call ReleaseSysCache(), or read directly from disk buffers, which go
away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), in the
heap_fetch() case. Or it may be a palloc'ed tuple, that you must
pfree() when finished.
10) What is elog()?
@ -429,3 +424,12 @@ c-mode)
to be broken into pieces so each piece can see rows modified by
previous pieces. CommandCounterIncrement() increments the Command
Counter, creating a new part of the transaction.
14) Why don't we use threads in the backend?
There are several reasons threads are not used:
* Historically, threads were unsupported and buggy.
* An error in one backend can corrupt other backends.
* Speed improvements using threads are small compared to the
remaining backend startup time.
* The backend code would be more complex.

871
doc/src/FAQ/FAQ_DEV.html
View file

@ -1,213 +1,229 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<TITLE>PostgreSQL Developers FAQ</title>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1>
Developer's Frequently Asked Questions (FAQ) for PostgreSQL
</H1>
<P>
Last updated: Fri Jun 9 21:54:54 EDT 2000
<P>
Current maintainer: Bruce Momjian (<a
href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<BR>
<P>
The most recent version of this document can be viewed at
the postgreSQL Web site, <a
href="http://PostgreSQL.org">http://PostgreSQL.org</a>.
<P>
<HR>
<P>
<HEAD>
<META name="generator" content="HTML Tidy, see www.w3.org">
<CENTER><H2>Questions</H2></CENTER>
<a href="#1">1</a>) What tools are available for developers?<BR>
<a href="#2">2</a>) What books are good for developers?<BR>
<a href="#3">3</a>) Why do we use <I>palloc</I>() and <I>pfree</I>() to allocate memory?<BR>
<a href="#4">4</a>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?<BR>
<a href="#5">5</a>) How do I add a feature or fix a bug?<BR>
<a href="#6">6</a>) How do I download/update the current source tree?<BR>
<a href="#7">7</a>) How do I test my changes?<BR>
<a href="#7">7</a>) I just added a field to a structure. What else
should I do?<BR>
<a href="#8">8</a>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I><BR>
<a href="#9">9</a>) How do I efficiently access information in
tables from the backend code?<BR>
<a href="#10">10</a>) What is elog()?<BR>
<a href="#11">11</a>) What is configure all about?<BR>
<a href="#12">12</a>) How do I add a new port?<BR>
<a href="#13">13</a>) What is CommandCounterIncrement()?<BR>
<BR>
<HR>
<TITLE>PostgreSQL Developers FAQ</TITLE>
</HEAD>
<H3><a
name="1">1</a>) What tools are available for developers?</H3><P>
<BODY bgcolor="#FFFFFF" text="#000000" link="#FF0000" vlink="#A00000"
alink="#0000FF">
<H1>Developer's Frequently Asked Questions (FAQ) for
PostgreSQL</H1>
Aside from the User documentation mentioned in the regular FAQ, there
are several development tools available. First, all the files in the
<I>/tools</I> directory are designed for developers.
<P>Last updated: Fri Jun 9 21:54:54 EDT 2000</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
</P>
<P>The most recent version of this document can be viewed at the
postgreSQL Web site, <A href=
"http://PostgreSQL.org">http://PostgreSQL.org</A>.<BR>
</P>
<HR>
<BR>
<CENTER>
<H2>Questions</H2>
</CENTER>
<A href="#1">1</A>) What tools are available for developers?<BR>
<A href="#2">2</A>) What books are good for developers?<BR>
<A href="#3">3</A>) Why do we use <I>palloc</I>() and
<I>pfree</I>() to allocate memory?<BR>
<A href="#4">4</A>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?<BR>
<A href="#5">5</A>) How do I add a feature or fix a bug?<BR>
<A href="#6">6</A>) How do I download/update the current source
tree?<BR>
<A href="#7">7</A>) How do I test my changes?<BR>
<A href="#7">7</A>) I just added a field to a structure. What else
should I do?<BR>
<A href="#8">8</A>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I><BR>
<A href="#9">9</A>) How do I efficiently access information in
tables from the backend code?<BR>
<A href="#10">10</A>) What is elog()?<BR>
<A href="#11">11</A>) What is configure all about?<BR>
<A href="#12">12</A>) How do I add a new port?<BR>
<A href="#13">13</A>) What is CommandCounterIncrement()?<BR>
<A href="#14">13</A>) Why don't we use threads in the backend?<BR>
<BR>
<HR>
<H3><A name="1">1</A>) What tools are available for
developers?</H3>
<P>Aside from the User documentation mentioned in the regular FAQ,
there are several development tools available. First, all the files
in the <I>/tools</I> directory are designed for developers.</P>
<PRE>
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directories
ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static
find_typedef get a list of typedefs in the source code
make_ctags make vi 'tags' file in each directory
make_diff make *.orig and diffs of source
make_etags make emacs 'etags' files
make_keywords.README make comparison of our keywords and SQL'92
make_mkid make mkid ID files
mkldexport create AIX exports file
pgindent indents C source files
pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directories
ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static
find_typedef get a list of typedefs in the source code
make_ctags make vi 'tags' file in each directory
make_diff make *.orig and diffs of source
make_etags make emacs 'etags' files
make_keywords.README make comparison of our keywords and SQL'92
make_mkid make mkid ID files
mkldexport create AIX exports file
pgindent indents C source files
pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog
</PRE>
Let me note some of these. If you point your browser at the
<I>file:/usr/local/src/pgsql/src/tools/backend/index.html</I>
directory, you will see few paragraphs describing the data flow,
the backend components in a flow chart, and a description of the
shared memory area. You can click on any flowchart box to see a
description. If you then click on the directory name, you will be
taken to the source directory, to browse the actual source code
behind it. We also have several README files in some source
directories to describe the function of the module. The browser
will display these when you enter the directory also. The
<I>tools/backend</I> directory is also contained on our web page
under the title <I>How PostgreSQL Processes a Query.</I>
Let me note some of these. If you point your browser at the
<I>file:/usr/local/src/pgsql/src/tools/backend/index.html</I> directory,
you will see few paragraphs describing the data flow, the backend
components in a flow chart, and a description of the shared memory area.
You can click on any flowchart box to see a description. If you then
click on the directory name, you will be taken to the source directory,
to browse the actual source code behind it. We also have several README
files in some source directories to describe the function of the module.
The browser will display these when you enter the directory also. The
<I>tools/backend</I> directory is also contained on our web page under
the title <I>How PostgreSQL Processes a Query.</I><P>
<P>Second, you really should have an editor that can handle tags,
so you can tag a function call to see the function definition, and
then tag inside that function to see an even lower-level function,
and then back out twice to return to the original function. Most
editors support this via <I>tags</I> or <I>etags</I> files.</P>
Second, you really should have an editor that can handle tags, so you
can tag a function call to see the function definition, and then tag
inside that function to see an even lower-level function, and then back
out twice to return to the original function. Most editors support this
via <I>tags</I> or <I>etags</I> files.<P>
Third, you need to get <I>id-utils</I> from:
<pre>
<a href="ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz">ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz</a>
<a href="ftp://tug.org/gnu/id-utils-3.2d.tar.gz">ftp://tug.org/gnu/id-utils-3.2d.tar.gz</a>
<a href="ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz">ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz</a>
</pre>
By running <I>tools/make_mkid</I>, an archive of source symbols can be
created that can be rapidly queried like <I>grep</I> or edited. Others
prefer <I>glimpse.</I><P>
<I>make_diff</I> has tools to create patch diff files that can be
applied to the distribution.<P>
Our standard format is to indent each code level with one tab, where
each tab is four spaces. You will need to set your editor to display
tabs as four spaces:
<BR>
<P>Third, you need to get <I>id-utils</I> from:</P>
<PRE>
vi in ~/.exrc:
set tabstop=4
set sw=4
more:
more -x4
less:
less -x4
emacs:
M-x set-variable tab-width
or
; Cmd to set tab stops &etc for working with PostgreSQL code
<A href=
"ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz">ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz</A>
<A href=
"ftp://tug.org/gnu/id-utils-3.2d.tar.gz">ftp://tug.org/gnu/id-utils-3.2d.tar.gz</A>
<A href=
"ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz">ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz</A>
</PRE>
By running <I>tools/make_mkid</I>, an archive of source symbols can
be created that can be rapidly queried like <I>grep</I> or edited.
Others prefer <I>glimpse.</I>
<P><I>make_diff</I> has tools to create patch diff files that can
be applied to the distribution.</P>
<P>Our standard format is to indent each code level with one tab,
where each tab is four spaces. You will need to set your editor to
display tabs as four spaces:<BR>
</P>
<PRE>
vi in ~/.exrc:
set tabstop=4
set sw=4
more:
more -x4
less:
less -x4
emacs:
M-x set-variable tab-width
or
; Cmd to set tab stops &amp; indenting for working with PostgreSQL code
(c-add-style "pgsql"
'("bsd"
'("bsd"
(indent-tabs-mode . t)
(c-basic-offset . 4)
(tab-width . 4)
(c-offsets-alist .
(c-offsets-alist .
((case-label . +))))
t) ; t = set this mode on
and add this to your autoload list (modify file path in macro):
and add this to your autoload list (modify file path in macro):
(setq auto-mode-alist
(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
auto-mode-alist))
or
/*
* Local variables:
* tab-width: 4
* c-indent-level: 4
* c-basic-offset: 4
* End:
*/
(setq auto-mode-alist
(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
auto-mode-alist))
or
/*
* Local variables:
* tab-width: 4
* c-indent-level: 4
* c-basic-offset: 4
* End:
*/
</PRE>
<BR>
<I>pgindent</I> will the format code by specifying
flags to your operating system's utility <I>indent.</I><P>
<I>pgindent</I> is run on all source files just before each beta test
period. It auto-formats all source files to make them consistent.
Comment blocks that need specific line breaks should be formatted as
<I>block comments,</I> where the comment starts as
<CODE>/*------</CODE>. These comments will not be reformatted in any
way.
<BR>
<I>pgindent</I> will the format code by specifying flags to your
operating system's utility <I>indent.</I>
<I>pginclude</I> contains scripts used to add needed #include's to
include files, and removed unneeded #include's.
<P><I>pgindent</I> is run on all source files just before each beta
test period. It auto-formats all source files to make them
consistent. Comment blocks that need specific line breaks should be
formatted as <I>block comments,</I> where the comment starts as
<CODE>/*------</CODE>. These comments will not be reformatted in
any way. <I>pginclude</I> contains scripts used to add needed
#include's to include files, and removed unneeded #include's. When
adding system types, you will need to assign oids to them. There is
also a script called <I>unused_oids</I> in
<I>pgsql/src/include/catalog</I> that shows the unused oids.</P>
When adding system types, you will need to assign oids to them.
There is also a script called <I>unused_oids</I> in
<I>pgsql/src/include/catalog</I> that shows the unused oids.
<H3><A name="2">2</A>) What books are good for developers?</H3>
<H3><a name="2">2</a>) What books are good for developers?</H3><P>
<P>I have four good books, <I>An Introduction to Database
Systems,</I> by C.J. Date, Addison, Wesley, <I>A Guide to the SQL
Standard,</I> by C.J. Date, et. al, Addison, Wesley,
<I>Fundamentals of Database Systems,</I> by Elmasri and Navathe,
and <I>Transaction Processing,</I> by Jim Gray, Morgan,
Kaufmann</P>
I have four good books, <I>An Introduction to Database Systems,</I> by
C.J. Date, Addison, Wesley, <I>A Guide to the SQL Standard,</I> by C.J.
Date, et. al, Addison, Wesley, <I>Fundamentals of Database Systems,</I>
by Elmasri and Navathe, and <I>Transaction Processing,</I> by Jim Gray,
Morgan, Kaufmann<P>
<P>There is also a database performance site, with a handbook
on-line written by Jim Gray at <A href=
"http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A></P>
There is also a database performance site, with a handbook on-line
written by Jim Gray at <A
HREF="http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A>
<H3><A name="3">3</A>) Why do we use <I>palloc</I>() and
<I>pfree</I>() to allocate memory?</H3>
<P><I>palloc()</I> and <I>pfree()</I> are used in place of malloc()
and free() because we automatically free all memory allocated when
a transaction completes. This makes it easier to make sure we free
memory that gets allocated in one place, but only freed much later.
There are several contexts that memory can be allocated in, and
this controls when the allocated memory is automatically freed by
the backend.</P>
<H3><A name="4">4</A>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?</H3>
<H3><a name="3">3</a>) Why do we use <I>palloc</I>() and <I>pfree</I>()
to allocate memory?</H3><P>
<P>We do this because this allows a consistent way to pass data
inside the backend in a flexible way. Every node has a
<I>NodeTag</I> which specifies what type of data is inside the
Node. <I>Lists</I> are groups of <I>Nodes chained together as a
forward-linked list.</I></P>
<I>palloc()</I> and <I>pfree()</I> are used in place of malloc() and
free() because we automatically free all memory allocated when a
transaction completes. This makes it easier to make sure we free memory
that gets allocated in one place, but only freed much later. There are
several contexts that memory can be allocated in, and this controls when
the allocated memory is automatically freed by the backend.<P>
<P>Here are some of the <I>List</I> manipulation commands:</P>
<BLOCKQUOTE>
<DL>
<DT>lfirst(i)</DT>
<H3><a name="4">4</a>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?</H3><P>
<DD>return the data at list element <I>i.</I></DD>
We do this because this allows a consistent way to pass data inside the
backend in a flexible way. Every node has a <I>NodeTag</I> which
specifies what type of data is inside the Node. <I>Lists</I> are groups
of <I>Nodes chained together as a forward-linked list.</I><P>
Here are some of the <I>List</I> manipulation commands:
<BLOCKQUOTE>
<DL>
<DT>lfirst(i)
<DD>return the data at list element <I>i.</I>
<DT>lnext(i)
<DD>return the next list element after <I>i.</I>
<DT>foreach(i, list)
<DD>loop through <I>list,</I> assigning each list element to <I>i.</I>
It is important to note that <I>i</I> is a List *, not the data in the
<I>List</I> element. You need to use <I>lfirst(i)</I> to get at the data.
Here is a typical code snipped that loops through a List containing
<I>Var *'s</I> and processes each one:
<DT>lnext(i)</DT>
<DD>return the next list element after <I>i.</I></DD>
<DT>foreach(i, list)</DT>
<DD>
loop through <I>list,</I> assigning each list element to
<I>i.</I> It is important to note that <I>i</I> is a List *,
not the data in the <I>List</I> element. You need to use
<I>lfirst(i)</I> to get at the data. Here is a typical code
snipped that loops through a List containing <I>Var *'s</I>
and processes each one:
<PRE>
<CODE>
List *i, *list;
<CODE>List *i, *list;
foreach(i, list)
{
@ -216,282 +232,307 @@ Here is a typical code snipped that loops through a List containing
/* process var here */
}
</CODE>
</PRE>
<DT>lcons(node, list)
<DD>add <I>node</I> to the front of <I>list,</I> or create a new list with
<I>node</I> if <I>list</I> is <I>NIL.</I>
<DT>lappend(list, node)
<DD>add <I>node</I> to the end of <I>list.</I> This is more expensive
that lcons.
<DT>nconc(list1, list2)
<DD>Concat <I>list2</I> on to the end of <I>list1.</I>
<DT>length(list)
<DD>return the length of the <I>list.</I>
<DT>nth(i, list)
<DD>return the <I>i</I>'th element in <I>list.</I>
<DT>lconsi, ...
<DD>There are integer versions of these: <I>lconsi, lappendi, nthi.</I>
<I>List's</I> containing integers instead of Node pointers are used to
hold list of relation object id's and other integer quantities.
</DL>
</BLOCKQUOTE>
You can print nodes easily inside <I>gdb.</I> First, to disable
output truncation when you use the gdb <I>print</I> command:
</PRE>
</DD>
<DT>lcons(node, list)</DT>
<DD>add <I>node</I> to the front of <I>list,</I> or create a
new list with <I>node</I> if <I>list</I> is <I>NIL.</I></DD>
<DT>lappend(list, node)</DT>
<DD>add <I>node</I> to the end of <I>list.</I> This is more
expensive that lcons.</DD>
<DT>nconc(list1, list2)</DT>
<DD>Concat <I>list2</I> on to the end of <I>list1.</I></DD>
<DT>length(list)</DT>
<DD>return the length of the <I>list.</I></DD>
<DT>nth(i, list)</DT>
<DD>return the <I>i</I>'th element in <I>list.</I></DD>
<DT>lconsi, ...</DT>
<DD>There are integer versions of these: <I>lconsi, lappendi,
nthi.</I> <I>List's</I> containing integers instead of Node
pointers are used to hold list of relation object id's and
other integer quantities.</DD>
</DL>
</BLOCKQUOTE>
You can print nodes easily inside <I>gdb.</I> First, to disable
output truncation when you use the gdb <I>print</I> command:
<PRE>
<CODE>
(gdb) set print elements 0
<CODE>(gdb) set print elements 0
</CODE>
</PRE>
Instead of printing values in gdb format, you can use the next two
commands to print out List, Node, and structure contents in a verbose
format that is easier to understand. List's are unrolled into nodes,
and nodes are printed in detail. The first prints in a short format,
and the second in a long format:
Instead of printing values in gdb format, you can use the next two
commands to print out List, Node, and structure contents in a
verbose format that is easier to understand. List's are unrolled
into nodes, and nodes are printed in detail. The first prints in a
short format, and the second in a long format:
<PRE>
<CODE>
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
<CODE>(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
</CODE>
</PRE>
The output appears in the postmaster log file, or on your screen if you
are running a backend directly without a postmaster.
<P>
The output appears in the postmaster log file, or on your screen if
you are running a backend directly without a postmaster.
<H3><a name="5">5</a>) How do I add a feature or fix a bug?</H3><P>
<H3><A name="5">5</A>) How do I add a feature or fix a bug?</H3>
The source code is over 250,000 lines. Many problems/features are
isolated to one specific area of the code. Others require knowledge of
much of the source. If you are confused about where to start, ask the
hackers list, and they will be glad to assess the complexity and give
pointers on where to start.<P>
<P>The source code is over 250,000 lines. Many problems/features
are isolated to one specific area of the code. Others require
knowledge of much of the source. If you are confused about where to
start, ask the hackers list, and they will be glad to assess the
complexity and give pointers on where to start.</P>
Another thing to keep in mind is that many fixes and features can be
added with surprisingly little code. I often start by adding code, then
looking at other areas in the code where similar things are done, and by
the time I am finished, the patch is quite small and compact.<P>
<P>Another thing to keep in mind is that many fixes and features
can be added with surprisingly little code. I often start by adding
code, then looking at other areas in the code where similar things
are done, and by the time I am finished, the patch is quite small
and compact.</P>
When adding code, keep in mind that it should use the existing
facilities in the source, for performance reasons and for simplicity.
Often a review of existing code doing similar things is helpful.<P>
<P>When adding code, keep in mind that it should use the existing
facilities in the source, for performance reasons and for
simplicity. Often a review of existing code doing similar things is
helpful.</P>
<H3><A name="6">6</A>) How do I download/update the current source
tree?</H3>
<H3><a name="6">6</a>) How do I download/update the current source
tree?</H3><P>
<P>There are several ways to obtain the source tree. Occasional
developers can just get the most recent source tree snapshot from
ftp.postgresql.org. For regular developers, you can use CVS. CVS
allows you to download the source tree, then occasionally update
your copy of the source tree with any new changes. Using CVS, you
don't have to download the entire source each time, only the
changed files. Anonymous CVS does not allows developers to update
the remote source tree, though privileged developers can do this.
There is a CVS FAQ on our web site that describes how to use remote
CVS. You can also use CVSup, which has similarly functionality, and
is available from ftp.postgresql.org.</P>
<P>To update the source tree, there are two ways. You can generate
a patch against your current source tree, perhaps using the
make_diff tools mentioned above, and send them to the patches list.
They will be reviewed, and applied in a timely manner. If the patch
is major, and we are in beta testing, the developers may wait for
the final release before applying your patches.</P>
There are several ways to obtain the source tree. Occasional developers
can just get the most recent source tree snapshot from
ftp.postgresql.org. For regular developers, you can use CVS. CVS
allows you to download the source tree, then occasionally update your
copy of the source tree with any new changes. Using CVS, you don't have
to download the entire source each time, only the changed files.
Anonymous CVS does not allows developers to update the remote source
tree, though privileged developers can do this. There is a CVS FAQ on
our web site that describes how to use remote CVS. You can also use
CVSup, which has similarly functionality, and is available from
ftp.postgresql.org.<P>
<P>For hard-core developers, Marc(scrappy@postgresql.org) will give
you a Unix shell account on postgresql.org, so you can use CVS to
update the main source tree, or you can ftp your files into your
account, patch, and cvs install the changes directly into the
source tree.</P>
To update the source tree, there are two ways. You can generate a patch
against your current source tree, perhaps using the make_diff tools
mentioned above, and send them to the patches list. They will be
reviewed, and applied in a timely manner. If the patch is major, and we
are in beta testing, the developers may wait for the final release
before applying your patches.<P>
<H3><A name="6">6</A>) How do I test my changes?</H3>
For hard-core developers, Marc(scrappy@postgresql.org) will give you a
Unix shell account on postgresql.org, so you can use CVS to update the
main source tree, or you can ftp your files into your account, patch,
and cvs install the changes directly into the source tree. <P>
<P>First, use <I>psql</I> to make sure it is working as you expect.
Then run <I>src/test/regress</I> and get the output of
<I>src/test/regress/checkresults</I> with and without your changes,
to see that your patch does not change the regression test in
unexpected ways. This practice has saved me many times. The
regression tests test the code in ways I would never do, and has
caught many bugs in my patches. By finding the problems now, you
save yourself a lot of debugging later when things are broken, and
you can't figure out when it happened.</P>
<H3><a name="6">6</a>) How do I test my changes?</H3><P>
<H3><A name="7">7</A>) I just added a field to a structure. What
else should I do?</H3>
First, use <I>psql</I> to make sure it is working as you expect. Then
run <I>src/test/regress</I> and get the output of
<I>src/test/regress/checkresults</I> with and without your changes, to
see that your patch does not change the regression test in unexpected
ways. This practice has saved me many times. The regression tests test
the code in ways I would never do, and has caught many bugs in my
patches. By finding the problems now, you save yourself a lot of
debugging later when things are broken, and you can't figure out when it
happened.<P>
<P>The structures passing around from the parser, rewrite,
optimizer, and executor require quite a bit of support. Most
structures have support routines in <I>src/backend/nodes</I> used
to create, copy, read, and output those structures. Make sure you
add support for your new field to these files. Find any other
places the structure may need code for your new field. <I>mkid</I>
is helpful with this (see above).</P>
<H3><A name="8">8</A>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I></H3>
<H3><a name="7">7</a>) I just added a field to a structure. What else
should I do?</H3><P>
The structures passing around from the parser, rewrite, optimizer, and
executor require quite a bit of support. Most structures have support
routines in <I>src/backend/nodes</I> used to create, copy, read, and output
those structures. Make sure you add support for your new field to these
files. Find any other places the structure may need code for your new
field. <I>mkid</I> is helpful with this (see above).<P>
<H3><a name="8">8</a>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I></H3><P>
Table, column, type, function, and view names are stored in system
tables in columns of type <I>Name.</I> Name is a fixed-length,
null-terminated type of <I>NAMEDATALEN</I> bytes. (The default value
for NAMEDATALEN is 32 bytes.)
<PRE><CODE>
typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
</CODE></PRE>
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length, null-terminated
character strings.<P>
Many functions are called with both types of names, ie. <I>heap_open().</I>
Because the Name type is null-terminated, it is safe to pass it to a
function expecting a char *. Because there are many cases where on-disk
names(Name) are compared to user-supplied names(char *), there are many
cases where Name and char * are used interchangeably.<P>
<H3><a name="9">9</a>) How do I efficiently access information in
tables from the backend code?</H3><P>
You first need to find the tuples(rows) you are interested in. There
are two ways. First, <I>SearchSysCache()</I> and related functions
allow you to query the system catalogs. This is the preferred way to
access system tables, because the first call to the cache loads the
needed rows, and future requests can return the results without
accessing the base table. The caches use system table indexes
to look up tuples. A list of available caches is located in
<I>src/backend/utils/cache/syscache.c.</I>
<I>src/backend/utils/cache/lsyscache.c</I> contains many column-specific
cache lookup functions.<P>
The rows returned are cache-owned versions of the heap rows. Therefore, you
must not modify or delete the tuple returned by <I>SearchSysCache()</I>. What
you <I>should</I> do is release it with <I>ReleaseSysCache()</I> when you are
done using it; this informs the cache that it can discard that tuple if
necessary. If you neglect to call <I>ReleaseSysCache()</I>, then the cache
entry will remain locked in the cache until end of transaction, which is
tolerable but not very desirable.<P>
If you can't use the system cache, you will need to retrieve the data
directly from the heap table, using the buffer cache that is shared by
all backends. The backend automatically takes care of loading the rows
into the buffer cache.<P>
Open the table with <I>heap_open().</I> You can then start a table scan
with <I>heap_beginscan(),</I> then use <I>heap_getnext()</I> and
continue as long as <I>HeapTupleIsValid()</I> returns true. Then do a
<I>heap_endscan().</I> <I>Keys</I> can be assigned to the <I>scan.</I>
No indexes are used, so all rows are going to be compared to the keys,
and only the valid rows returned.<P>
You can also use <I>heap_fetch()</I> to fetch rows by block
number/offset. While scans automatically lock/unlock rows from the
buffer cache, with <I>heap_fetch(),</I> you must pass a <I>Buffer</I>
pointer, and <I>ReleaseBuffer()</I> it when completed.<P>
Once you have the row, you can get data that is common to all tuples,
like <I>t_self</I> and <I>t_oid,</I> by merely accessing the
<I>HeapTuple</I> structure entries.
If you need a table-specific column, you should take the HeapTuple
pointer, and use the <I>GETSTRUCT()</I> macro to access the
table-specific start of the tuple. You then cast the pointer as a
<I>Form_pg_proc</I> pointer if you are accessing the pg_proc table, or
<I>Form_pg_type</I> if you are accessing pg_type. You can then access
the columns by using a structure pointer:
<P>Table, column, type, function, and view names are stored in
system tables in columns of type <I>Name.</I> Name is a
fixed-length, null-terminated type of <I>NAMEDATALEN</I> bytes.
(The default value for NAMEDATALEN is 32 bytes.)</P>
<PRE>
<CODE>
((Form_pg_class) GETSTRUCT(tuple))-&gt;relnatts
<CODE>typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
</CODE>
</PRE>
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length,
null-terminated character strings.
You must not directly change <I>live</I> tuples in this way. The best
way is to use <I>heap_modifytuple()</I> and pass it your original
tuple, and the values you want changed. It returns a palloc'ed
tuple, which you pass to <I>heap_replace().</I>
<P>Many functions are called with both types of names, ie.
<I>heap_open().</I> Because the Name type is null-terminated, it is
safe to pass it to a function expecting a char *. Because there are
many cases where on-disk names(Name) are compared to user-supplied
names(char *), there are many cases where Name and char * are used
interchangeably.</P>
You can delete tuples by passing the tuple's <I>t_self</I> to
<I>heap_destroy().</I> You use <I>t_self</I> for <I>heap_update()</I> too.
<H3><A name="9">9</A>) How do I efficiently access information in
tables from the backend code?</H3>
Remember, tuples can be either system cache copies, which may go away after
you call <I>ReleaseSysCache()</I>, or read directly from disk buffers, which
go away when you <I>heap_getnext()</I>, <I>heap_endscan</I>, or
<I>ReleaseBuffer()</I>, in the <I>heap_fetch()</I> case. Or it may be a
palloc'ed tuple, that you must <I>pfree()</I> when finished.
<P>You first need to find the tuples(rows) you are interested in.
There are two ways. First, <I>SearchSysCache()</I> and related
functions allow you to query the system catalogs. This is the
preferred way to access system tables, because the first call to
the cache loads the needed rows, and future requests can return the
results without accessing the base table. The caches use system
table indexes to look up tuples. A list of available caches is
located in <I>src/backend/utils/cache/syscache.c.</I>
<I>src/backend/utils/cache/lsyscache.c</I> contains many
column-specific cache lookup functions.</P>
<H3><a name="10">10</a>) What is elog()?</H3><P>
<P>The rows returned are cache-owned versions of the heap rows.
Therefore, you must not modify or delete the tuple returned by
<I>SearchSysCache()</I>. What you <I>should</I> do is release it
with <I>ReleaseSysCache()</I> when you are done using it; this
informs the cache that it can discard that tuple if necessary. If
you neglect to call <I>ReleaseSysCache()</I>, then the cache entry
will remain locked in the cache until end of transaction, which is
tolerable but not very desirable.</P>
<I>elog()</I> is used to send messages to the front-end, and optionally
terminate the current query being processed. The first parameter is an
elog level of <I>NOTICE,</I> <I>DEBUG,</I> <I>ERROR,</I> or
<I>FATAL.</I>
<P>If you can't use the system cache, you will need to retrieve the
data directly from the heap table, using the buffer cache that is
shared by all backends. The backend automatically takes care of
loading the rows into the buffer cache.</P>
<I>NOTICE</I> prints on the user's terminal and the postmaster logs.
<I>DEBUG</I> prints only in the postmaster logs. <I>ERROR</I> prints in
both places, and terminates the current query, never returning from the call.
<I>FATAL</I> terminates the backend process.
<P>Open the table with <I>heap_open().</I> You can then start a
table scan with <I>heap_beginscan(),</I> then use
<I>heap_getnext()</I> and continue as long as
<I>HeapTupleIsValid()</I> returns true. Then do a
<I>heap_endscan().</I> <I>Keys</I> can be assigned to the
<I>scan.</I> No indexes are used, so all rows are going to be
compared to the keys, and only the valid rows returned.</P>
The remaining parameters of <I>elog</I> are a <I>printf</I>-style set of
parameters to print.
<P>You can also use <I>heap_fetch()</I> to fetch rows by block
number/offset. While scans automatically lock/unlock rows from the
buffer cache, with <I>heap_fetch(),</I> you must pass a
<I>Buffer</I> pointer, and <I>ReleaseBuffer()</I> it when
completed.</P>
<H3><a name="11">11</a>) What is configure all about?</H3><P>
<P>Once you have the row, you can get data that is common to all
tuples, like <I>t_self</I> and <I>t_oid,</I> by merely accessing
the <I>HeapTuple</I> structure entries. If you need a
table-specific column, you should take the HeapTuple pointer, and
use the <I>GETSTRUCT()</I> macro to access the table-specific start
of the tuple. You then cast the pointer as a <I>Form_pg_proc</I>
pointer if you are accessing the pg_proc table, or
<I>Form_pg_type</I> if you are accessing pg_type. You can then
access the columns by using a structure pointer:</P>
<PRE>
<CODE>((Form_pg_class) GETSTRUCT(tuple))-&gt;relnatts
</CODE>
</PRE>
You must not directly change <I>live</I> tuples in this way. The
best way is to use <I>heap_modifytuple()</I> and pass it your
original tuple, and the values you want changed. It returns a
palloc'ed tuple, which you pass to <I>heap_replace().</I> You can
delete tuples by passing the tuple's <I>t_self</I> to
<I>heap_destroy().</I> You use <I>t_self</I> for
<I>heap_update()</I> too. Remember, tuples can be either system
cache copies, which may go away after you call
<I>ReleaseSysCache()</I>, or read directly from disk buffers, which
go away when you <I>heap_getnext()</I>, <I>heap_endscan</I>, or
<I>ReleaseBuffer()</I>, in the <I>heap_fetch()</I> case. Or it may
be a palloc'ed tuple, that you must <I>pfree()</I> when finished.
The files <I>configure</I> and <I>configure.in</I> are part of the
GNU <I>autoconf</I> package. Configure allows us to test for various
capabilities of the OS, and to set variables that can then be tested in
C programs and Makefiles. Autoconf is installed on the PostgreSQL main
server. To add options to configure, edit <I>configure.in,</I> and then
run <I>autoconf</I> to generate <I>configure.</I><P>
<H3><A name="10">10</A>) What is elog()?</H3>
When <I>configure</I> is run by the user, it tests various OS
capabilities, stores those in <I>config.status</I> and
<I>config.cache,</I> and modifies a list of <I>*.in</I> files. For
example, if there exists a <I>Makefile.in,</I> configure generates a
<I>Makefile</I> that contains substitutions for all @var@ parameters
found by configure.<P>
<P><I>elog()</I> is used to send messages to the front-end, and
optionally terminate the current query being processed. The first
parameter is an elog level of <I>NOTICE,</I> <I>DEBUG,</I>
<I>ERROR,</I> or <I>FATAL.</I> <I>NOTICE</I> prints on the user's
terminal and the postmaster logs. <I>DEBUG</I> prints only in the
postmaster logs. <I>ERROR</I> prints in both places, and terminates
the current query, never returning from the call. <I>FATAL</I>
terminates the backend process. The remaining parameters of
<I>elog</I> are a <I>printf</I>-style set of parameters to
print.</P>
When you need to edit files, make sure you don't waste time modifying
files generated by <I>configure.</I> Edit the <I>*.in</I> file, and
re-run <I>configure</I> to recreate the needed file. If you run <I>make
distclean</I> from the top-level source directory, all files derived by
configure are removed, so you see only the file contained in the source
distribution.<P>
<H3><A name="11">11</A>) What is configure all about?</H3>
<H3><a name="12">12</a>) How do I add a new port?</H3><P>
<P>The files <I>configure</I> and <I>configure.in</I> are part of
the GNU <I>autoconf</I> package. Configure allows us to test for
various capabilities of the OS, and to set variables that can then
be tested in C programs and Makefiles. Autoconf is installed on the
PostgreSQL main server. To add options to configure, edit
<I>configure.in,</I> and then run <I>autoconf</I> to generate
<I>configure.</I></P>
There are a variety of places that need to be modified to add a new
port. First, start in the <I>src/template</I> directory. Add an
appropriate entry for your OS. Also, use <I>src/config.guess</I> to add
your OS to <I>src/template/.similar.</I> You shouldn't match the OS
version exactly. The <I>configure</I> test will look for an exact OS
version number, and if not found, find a match without version number.
Edit <I>src/configure.in</I> to add your new OS. (See configure item
above.) You will need to run autoconf, or patch <I>src/configure</I>
too.<P>
<P>When <I>configure</I> is run by the user, it tests various OS
capabilities, stores those in <I>config.status</I> and
<I>config.cache,</I> and modifies a list of <I>*.in</I> files. For
example, if there exists a <I>Makefile.in,</I> configure generates
a <I>Makefile</I> that contains substitutions for all @var@
parameters found by configure.</P>
Then, check <I>src/include/port</I> and add your new OS file, with
appropriate values. Hopefully, there is already locking code in
<I>src/include/storage/s_lock.h</I> for your CPU. There is also a
<I>src/makefiles</I> directory for port-specific Makefile handling.
There is a <I>backend/port</I> directory if you need special files for
your OS.<P>
<P>When you need to edit files, make sure you don't waste time
modifying files generated by <I>configure.</I> Edit the <I>*.in</I>
file, and re-run <I>configure</I> to recreate the needed file. If
you run <I>make distclean</I> from the top-level source directory,
all files derived by configure are removed, so you see only the
file contained in the source distribution.</P>
<H3><a name="13">13</a>) What is CommandCounterIncrement()?</H3><P>
<H3><A name="12">12</A>) How do I add a new port?</H3>
Normally, transactions can not see the rows they modify. This allows <CODE>
UPDATE foo SET x = x + 1</CODE> to work correctly.
<P>
<P>There are a variety of places that need to be modified to add a
new port. First, start in the <I>src/template</I> directory. Add an
appropriate entry for your OS. Also, use <I>src/config.guess</I> to
add your OS to <I>src/template/.similar.</I> You shouldn't match
the OS version exactly. The <I>configure</I> test will look for an
exact OS version number, and if not found, find a match without
version number. Edit <I>src/configure.in</I> to add your new OS.
(See configure item above.) You will need to run autoconf, or patch
<I>src/configure</I> too.</P>
However, there are cases where a transactions needs to see rows affected
in previous parts of the transaction. This is accomplished using a
Command Counter. Incrementing the counter allows transactions to be
broken into pieces so each piece can see rows modified by previous
pieces. <I>CommandCounterIncrement()</I> increments the Command
Counter, creating a new part of the transaction. <P>
<P>Then, check <I>src/include/port</I> and add your new OS file,
with appropriate values. Hopefully, there is already locking code
in <I>src/include/storage/s_lock.h</I> for your CPU. There is also
a <I>src/makefiles</I> directory for port-specific Makefile
handling. There is a <I>backend/port</I> directory if you need
special files for your OS.</P>
</BODY>
<H3><A name="13">13</A>) What is CommandCounterIncrement()?</H3>
<P>Normally, transactions can not see the rows they modify. This
allows <CODE>UPDATE foo SET x = x + 1</CODE> to work correctly.</P>
<P>However, there are cases where a transactions needs to see rows
affected in previous parts of the transaction. This is accomplished
using a Command Counter. Incrementing the counter allows
transactions to be broken into pieces so each piece can see rows
modified by previous pieces. <I>CommandCounterIncrement()</I>
increments the Command Counter, creating a new part of the
transaction.</P>
<H3><A name="14">14</A>) Why don't we use threads in the
backend?</H3>
<P>There are several reasons threads are not used:</P>
<UL>
<LI>Historically, threads were unsupported and buggy.</LI>
<LI>An error in one backend can corrupt other backends.</LI>
<LI>Speed improvements using threads are small compared to the
remaining backend startup time.</LI>
<LI>The backend code would be more complex.</LI>
</UL>
</BODY>
</HTML>