open-dsi
/
postgres
mirror of https://github.com/postgres/postgres
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Update backend flowchard wording

Browse Source
REL8_1_STABLE
Bruce Momjian 21 years ago
parent 6ceebcac3a
commit 8903592b10
1 changed files with 167 additions and 162 deletions
Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL
Show Stats Download Patch File Download Diff File
  1. 329
      src/tools/backend/index.html

329
src/tools/backend/index.html
Unescape View File

@ -1,162 +1,167 @@
<HTML>
<HEAD>
<TITLE>How PostgreSQL Processes a Query</TITLE>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1>
How PostgreSQL Processes a Query
</H1>
<H2>
by Bruce Momjian
</H2>
<P>
<IMG src="flow.gif" usemap="#flowmap" alt="flowchart">
<MAP name="flowmap" id="flowmap">
<AREA coords="125,35,245,65" HREF="backend_dirs.html#main" alt="main"></AREA>
<AREA coords="125,100,245,125" HREF="backend_dirs.html#postmaster" alt="postmaster"></AREA>
<AREA coords="325,65,450,95" HREF="backend_dirs.html#libpq" alt="libpq"></AREA>
<AREA coords="125,160,245,190" HREF="backend_dirs.html#tcop" alt="tcop"></AREA>
<AREA coords="325,160,450,190" HREF="backend_dirs.html#tcop" alt="tcop"></AREA>
<AREA coords="125,240,245,265" HREF="backend_dirs.html#parser" alt="parser"></AREA>
<AREA coords="125,300,250,330" HREF="backend_dirs.html#tcop" alt="tcop"></AREA>
<AREA coords="125,360,250,390" HREF="backend_dirs.html#optimizer" alt="optimizer"></AREA>
<AREA coords="125,425,245,455" HREF="backend_dirs.html#optimizer_plan" alt="plan"></AREA>
<AREA coords="125,490,245,515" HREF="backend_dirs.html#executor" alt="executor"></AREA>
<AREA coords="325,300,450,330" HREF="backend_dirs.html#commands" alt="commands"></AREA>
<AREA coords="75,575,195,605" HREF="backend_dirs.html#utils" alt="utils"></AREA>
<AREA coords="235,575,360,605" HREF="backend_dirs.html#catalog" alt="catalog"></AREA>
<AREA coords="405,575,525,605" HREF="backend_dirs.html#storage" alt="storage"></AREA>
<AREA coords="155,635,275,665" HREF="backend_dirs.html#access" alt="access"></AREA>
<AREA coords="325,635,450,665" HREF="backend_dirs.html#nodes" alt="nodes"></AREA>
<AREA coords="75,705,200,730" HREF="backend_dirs.html#bootstrap" alt="bootstrap"></AREA>
</MAP>
<EM>
Click on an item to see more detail or look at the full
<A HREF="backend_dirs.html">index.</A>
</EM>
<BR>
<BR>
</P>
<P>
A query comes to the backend via data packets arriving through TCP/IP or
Unix Domain sockets. It is loaded into a string, and passed to the
<A HREF="../../backend/parser">parser,</A> where the lexical scanner,
<A HREF="../../backend/parser/scan.l">scan.l,</A> breaks the query up
into tokens(words). The parser uses <A
HREF="../../backend/parser/gram.y">gram.y</A> and the tokens to identify
the query type, and load the proper query-specific structure, like <A
HREF="../../include/nodes/parsenodes.h">CreateStmt</A> or <A
HREF="../../include/nodes/parsenodes.h">SelectStmt.</A></P><P>
The query is then identified as a <I>Utility</I> query or a more complex
query. A <I>Utility</I> query is processed by a query-specific function
in <A HREF="../../backend/commands"> commands.</A> A complex query, like
<I>SELECT, UPDATE,</I> and <I>DELETE</I> requires much more handling.</P><P>
The parser takes a complex query, and creates a
<A HREF="../../include/nodes/parsenodes.h">Query</A> structure that
contains all the elements used by complex queries. Query.qual holds the
<I>WHERE</I> clause qualification, which is filled in by <A
HREF="../../backend/parser/parse_clause.c">transformWhereClause().</A>
Each table referenced in the query is represented by a <A
HREF="../../include/nodes/parsenodes.h"> RangeTableEntry,</A> and they
are linked together to form the <I>range table</I> of the query, which
is generated by <A HREF="../../backend/parser/parse_clause.c">
transformFromClause().</A> Query.rtable holds the query's range table.</P><P>
Certain queries, like <I>SELECT,</I> return columns of data. Other
queries, like <I>INSERT</I> and <I>UPDATE,</I> specify the columns
modified by the query. These column references are converted to <A
HREF="../../include/nodes/primnodes.h">TargetEntry</A> entries, which are
linked together to make up the <I>target list</I> of
the query. The target list is stored in Query.targetList, which is
generated by <A
HREF="../../backend/parser/parse_target.c">transformTargetList().</A></P><P>
Other query elements, like aggregates(<I>SUM()</I>), <I>GROUP BY,</I>
and <I>ORDER BY</I> are also stored in their own Query fields.</P><P>
The next step is for the Query to be modified by any <I>VIEWS</I> or
<I>RULES</I> that may apply to the query. This is performed by the <A
HREF="../../backend/rewrite">rewrite</A> system.</P><P>
The <A HREF="../../backend/optimizer">optimizer</A> takes the Query
structure and generates an optimal <A
HREF="../../include/nodes/plannodes.h">Plan,</A> which contains the
operations to be performed to execute the query. The <A
HREF="../../backend/optimizer/path">path</A> module determines the best
table join order and join type of each table in the RangeTable, using
Query.qual(<I>WHERE</I> clause) to consider optimal index usage.</P><P>
The Plan is then passed to the <A
HREF="../../backend/executor">executor</A> for execution, and the result
returned to the client. The Plan actually as set of nodes, arranged in
a tree structure with a top-level node, and various sub-nodes as
children.</P><P>
There are many other modules that support this basic functionality. They
can be accessed by clicking on the flowchart.</P>
<HR><P>
Another area of interest is the shared memory area, which contains data
accessable to all backends. It has recently used data/index blocks,
locks, backend process information, and lookup tables for these
structures:
</P>
<UL>
<LI>ShmemIndex - lookup shared memory addresses using structure names</LI>
<LI><A HREF="../../include/storage/buf_internals.h">Buffer
Descriptor</A> - control header for buffer cache block</LI>
<LI><A HREF="../../include/storage/buf_internals.h">Buffer Block</A> -
data/index buffer cache block</LI>
<LI>Shared Buffer Lookup Table - lookup of buffer cache block addresses
using table name and block number(<A
HREF="../../include/storage/buf_internals.h"> BufferTag</A>)</LI>
<LI>MultiLevelLockTable (ctl) - control structure for each locking
method. Currently, only multi-level locking is used(<A
HREF="../../include/storage/lock.h">LOCKMETHODCTL</A>).</LI>
<LI>MultiLevelLockTable (lock hash) - the <A
HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using
relation, database object ids(<A
HREF="../../include/storage/lock.h">LOCKTAG)</A>. The lock table
structure contains the lock modes(read/write or shared/exclusive) and
circular linked list of backends (<A
HREF="../../include/storage/proc.h">PROC</A> structure pointers) waiting
on the lock.</LI>
<LI>MultiLevelLockTable (xid hash) - lookup of LOCK structure address
using transaction id, LOCK address. It is used to quickly check if the
current transaction already has any locks on a table, rather than having
to search through all the held locks. It also stores the modes
(read/write) of the locks held by the current transaction. The returned
<A HREF="../../include/storage/lock.h">XIDLookupEnt</A> structure also
contains a pointer to the backend's PROC.lockQueue.</LI>
<LI><A HREF="../../include/storage/proc.h">Proc Header</A> - information
about each backend, including locks held/waiting, indexed by process id</LI>
</UL>
<P>Each data structure is created by calling <A
HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and the
lookups are created by <A
HREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A></P>
<HR>
<SMALL>
Maintainer: Bruce Momjian (<A
HREF="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
Last updated: Mon Aug 10 10:48:06 EDT 1998
</SMALL>
</BODY>
</HTML>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta name="generator"
content="HTML Tidy for BSD/OS (vers 1st July 2002), see www.w3.org" />
<title>How PostgreSQL Processes a Query</title>
</head>
<body bgcolor="#FFFFFF" text="#000000" link="#FF0000"
vlink="#A00000" alink="#0000FF">
<h1>How PostgreSQL Processes a Query</h1>
<h2>by Bruce Momjian</h2>
<p><img src="flow.gif" usemap="#flowmap" alt="flowchart" />
<em>Click on an item to see more detail or look at the full
<a href="backend_dirs.html">index.</a></em>
<map name="flowmap" id="flowmap">
<area coords="125,35,245,65" href="backend_dirs.html#main" alt="main" />
<area coords="125,100,245,125" href="backend_dirs.html#postmaster" alt="postmaster" />
<area coords="325,65,450,95" href="backend_dirs.html#libpq" alt="libpq" />
<area coords="125,160,245,190" href="backend_dirs.html#tcop" alt="tcop" />
<area coords="325,160,450,190" href="backend_dirs.html#tcop" alt="tcop" />
<area coords="125,240,245,265" href="backend_dirs.html#parser" alt="parser" />
<area coords="125,300,250,330" href="backend_dirs.html#tcop" alt="tcop" />
<area coords="125,360,250,390" href="backend_dirs.html#optimizer" alt="optimizer" />
<area coords="125,425,245,455" href="backend_dirs.html#optimizer_plan" alt="plan" />
<area coords="125,490,245,515" href="backend_dirs.html#executor" alt="executor" />
<area coords="325,300,450,330" href="backend_dirs.html#commands" alt="commands" />
<area coords="75,575,195,605" href="backend_dirs.html#utils" alt="utils" />
<area coords="235,575,360,605" href="backend_dirs.html#catalog" alt="catalog" />
<area coords="405,575,525,605" href="backend_dirs.html#storage" alt="storage" />
<area coords="155,635,275,665" href="backend_dirs.html#access" alt="access" />
<area coords="325,635,450,665" href="backend_dirs.html#nodes" alt="nodes" />
<area coords="75,705,200,730" href="backend_dirs.html#bootstrap" alt="bootstrap" />
</map>
<br />
<p>A query comes to the backend via data packets arriving through
TCP/IP or Unix Domain sockets. It is loaded into a string, and
passed to the <a href="../../backend/parser">parser,</a> where the
lexical scanner, <a href="../../backend/parser/scan.l">scan.l,</a>
breaks the query up into tokens(words). The parser uses <a
href="../../backend/parser/gram.y">gram.y</a> and the tokens to
identify the query type, and load the proper query-specific
structure, like <a
href="../../include/nodes/parsenodes.h">CreateStmt</a> or <a
href="../../include/nodes/parsenodes.h">SelectStmt.</a></p>
<p>The statement is then identified as complex (<i>SELECT / INSERT /
UPDATE / DELETE</i>) or a simple, e.g <i> CREATE USER, ANALYZE, </i>,
etc. Utility commands are processed by statement-specific functions in <a
href="../../backend/commands">backend/commands.</a> Complex statements
require more handling.</p>
<p>The parser takes a complex query, and creates a <a
href="../../include/nodes/parsenodes.h">Query</a> structure that
contains all the elements used by complex queries. Query.qual holds
the <i>WHERE</i> clause qualification, which is filled in by <a
href="../../backend/parser/parse_clause.c">transformWhereClause().</a>
Each table referenced in the query is represented by a <a
href="../../include/nodes/parsenodes.h">RangeTableEntry,</a> and
they are linked together to form the <i>range table</i> of the
query, which is generated by <a
href="../../backend/parser/parse_clause.c">transformFromClause().</a>
Query.rtable holds the query's range table.</p>
<p>Certain queries, like <i>SELECT,</i> return columns of data.
Other queries, like <i>INSERT</i> and <i>UPDATE,</i> specify the
columns modified by the query. These column references are
converted to <a
href="../../include/nodes/primnodes.h">TargetEntry</a> entries,
which are linked together to make up the <i>target list</i> of the
query. The target list is stored in Query.targetList, which is
generated by <a
href="../../backend/parser/parse_target.c">transformTargetList().</a></p>
<p>Other query elements, like aggregates(<i>SUM()</i>), <i>GROUP
BY,</i> and <i>ORDER BY</i> are also stored in their own Query
fields.</p>
<p>The next step is for the Query to be modified by any
<i>VIEWS</i> or <i>RULES</i> that may apply to the query. This is
performed by the <a href="../../backend/rewrite">rewrite</a>
system.</p>
<p>The <a href="../../backend/optimizer">optimizer</a> takes the
Query structure and generates an optimal <a
href="../../include/nodes/plannodes.h">Plan,</a> which contains the
operations to be performed to execute the query. The <a
href="../../backend/optimizer/path">path</a> module determines the
best table join order and join type of each table in the
RangeTable, using Query.qual(<i>WHERE</i> clause) to consider
optimal index usage.</p>
<p>The Plan is then passed to the <a
href="../../backend/executor">executor</a> for execution, and the
result returned to the client. The Plan actually as set of nodes,
arranged in a tree structure with a top-level node, and various
sub-nodes as children.</p>
<p>There are many other modules that support this basic
functionality. They can be accessed by clicking on the
flowchart.</p>
<hr />
<p>Another area of interest is the shared memory area, which
contains data accessable to all backends. It has recently used
data/index blocks, locks, backend process information, and lookup
tables for these structures:</p>
<ul>
<li>ShmemIndex - lookup shared memory addresses using structure
names</li>
<li><a href="../../include/storage/buf_internals.h">Buffer
Descriptor</a> - control header for buffer cache block</li>
<li><a href="../../include/storage/buf_internals.h">Buffer
Block</a> - data/index buffer cache block</li>
<li>Shared Buffer Lookup Table - lookup of buffer cache block
addresses using table name and block number( <a
href="../../include/storage/buf_internals.h">BufferTag</a>)</li>
<li>MultiLevelLockTable (ctl) - control structure for each locking
method. Currently, only multi-level locking is used(<a
href="../../include/storage/lock.h">LOCKMETHODCTL</a>).</li>
<li>MultiLevelLockTable (lock hash) - the <a
href="../../include/storage/lock.h">LOCK</a> structure, looked up
using relation, database object ids(<a
href="../../include/storage/lock.h">LOCKTAG)</a>. The lock table
structure contains the lock modes(read/write or shared/exclusive)
and circular linked list of backends (<a
href="../../include/storage/proc.h">PROC</a> structure pointers)
waiting on the lock.</li>
<li>MultiLevelLockTable (xid hash) - lookup of LOCK structure
address using transaction id, LOCK address. It is used to quickly
check if the current transaction already has any locks on a table,
rather than having to search through all the held locks. It also
stores the modes (read/write) of the locks held by the current
transaction. The returned <a
href="../../include/storage/lock.h">XIDLookupEnt</a> structure also
contains a pointer to the backend's PROC.lockQueue.</li>
<li><a href="../../include/storage/proc.h">Proc Header</a> -
information about each backend, including locks held/waiting,
indexed by process id</li>
</ul>
<p>Each data structure is created by calling <a
href="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</a> and
the lookups are created by <a
href="../../backend/storage/ipc/shmem.c">ShmemInitHash().</a></p>
<hr />
<small>Maintainer: Bruce Momjian (<a
href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<br />
Last updated: Fri May 6 14:22:27 EDT 2005</small>
</body>
</html>

Write Preview
Loading…
Cancel
Save