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Bruce Momjian 1998-06-30 04:35:34 +00:00
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<HTML> <HTML>
<HEAD> <HEAD>
<TITLE>PostgreSQL Backend Flowchart</TITLE> <TITLE>How PostgreSQL Processes a Query</TITLE>
</HEAD> </HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF"> <BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1 ALIGN=CENTER> <H1 ALIGN=CENTER>
PostgreSQL Backend Flowchart How PostgreSQL Processes a Query
</H1> </H1>
<H2 ALIGN=CENTER> <H2 ALIGN=CENTER>
by Bruce Momjian by Bruce Momjian
</H2> </H2>
<P> <P>
A query come into the backend via data packets coming in through TCP/IP A query comes to the backend via data packets arriving through TCP/IP
and Unix Domain sockets. It is loaded into a string, and passed to or Unix Domain sockets. It is loaded into a string, and passed to
the the
<A HREF="../../backend/parser">parser,</A> where the lexical scanner, <A HREF="../../backend/parser">parser,</A> where the lexical scanner,
<A HREF="../../backend/parser/scan.l">scan.l,</A> <A HREF="../../backend/parser/scan.l">scan.l,</A>
breaks the query up into tokens(words). The parser breaks the query up into tokens(words). The parser
uses uses
<A HREF="../../backend/parser/gram.y">gram.y</A> and the tokens to <A HREF="../../backend/parser/gram.y">gram.y</A> and the tokens to
identify the query type, and load the proper query-specific identify the query type, and load the proper query-specific structure,
structure, like like <A HREF="../../include/nodes/parsenodes.h">CreateStmt</A> or <A
<A HREF="../../include/nodes/parsenodes.h">CreateStmt or SelectStmt.</A> HREF="../../include/nodes/parsenodes.h">SelectStmt.</A>
<P> <P>
The query is then identified as a <I>Utility</I> function or a more The query is then identified as a <I>Utility</I> query or a more complex
complex query. A <I>Utility</I> query is processed by a query. A <I>Utility</I> query is processed by a query-specific function
query-specific function in <A HREF="../../backend/commands"> in <A HREF="../../backend/commands"> commands.</A> A complex query, like
commands.</A> A complex query, like <CODE>SELECT, UPDATE,</CODE> and <CODE>SELECT, UPDATE,</CODE> and
<CODE>DELETE</CODE> requires much more handling. <CODE>DELETE</CODE> requires much more handling.
<P> <P>
The parser takes a complex query, and creates a The parser takes a complex query, and creates a
@ -37,7 +37,7 @@ Each table referenced in the query is represented by a <A
HREF="../../include/nodes/parsenodes.h"> RangeTableEntry,</A> and they HREF="../../include/nodes/parsenodes.h"> RangeTableEntry,</A> and they
are linked together to form the <I>range table</I> of the query, which is are linked together to form the <I>range table</I> of the query, which is
generated by <A HREF="../../backend/parser/parse_clause.c"> generated by <A HREF="../../backend/parser/parse_clause.c">
makeRangeTable().</A> Query.rtable holds the queries range table. makeRangeTable().</A> Query.rtable holds the query's range table.
<P> <P>
Certain queries, like <CODE>SELECT,</CODE> return columns of data. Other Certain queries, like <CODE>SELECT,</CODE> return columns of data. Other
queries, like <CODE>INSERT</CODE> and <CODE>UPDATE,</CODE> specify the columns queries, like <CODE>INSERT</CODE> and <CODE>UPDATE,</CODE> specify the columns
@ -48,7 +48,7 @@ target list is stored in Query.targetList, which is generated by
<A HREF="../../backend/parser/parse_target.c">transformTargetList().</A> <A HREF="../../backend/parser/parse_target.c">transformTargetList().</A>
<P> <P>
Other query elements, like aggregates(<CODE>SUM()</CODE>), <CODE>GROUP BY,</CODE> Other query elements, like aggregates(<CODE>SUM()</CODE>), <CODE>GROUP BY,</CODE>
<CODE>ORDER BY</CODE> are also stored in their own Query fields. and <CODE>ORDER BY</CODE> are also stored in their own Query fields.
<P> <P>
The next step is for the Query to be modified by any <CODE>VIEWS</CODE> or The next step is for the Query to be modified by any <CODE>VIEWS</CODE> or
<CODE>RULES</CODE> that may apply to the query. This is performed by the <A <CODE>RULES</CODE> that may apply to the query. This is performed by the <A
@ -69,21 +69,22 @@ returned to the client.
There are many other modules that support this basic functionality. There are many other modules that support this basic functionality.
They can be accessed by clicking on the flowchart. They can be accessed by clicking on the flowchart.
<P> <P>
Another area of interest is the shared memory area, containing table Another area of interest is the shared memory area, which contains data
data/index blocks, locks, backend information, and lookup tables for accessable to all backends. It has table recently used data/index
these structures: blocks, locks, backend information, and lookup tables for these
structures:
<UL> <UL>
<LI>ShmemIndex - lookup shared memory addresses using structure names <LI>ShmemIndex - lookup shared memory addresses using structure names
<LI><A HREF="../../include/storage/buf_internals.h">Buffer <LI><A HREF="../../include/storage/buf_internals.h">Buffer
Descriptors</A> - control header for buffer cache block Descriptor</A> - control header for buffer cache block
<LI><A HREF="../../include/storage/buf_internals.h">Buffer Blocks</A> - <LI><A HREF="../../include/storage/buf_internals.h">Buffer Block</A> -
data/index buffer cache block data/index buffer cache block
<LI>Shared Buf Lookup Table - lookup of buffer cache block address using <LI>Shared Buffer Lookup Table - lookup of buffer cache block addresses using
table name and block number(<A HREF="../../include/storage/buf_internals.h"> table name and block number(<A HREF="../../include/storage/buf_internals.h">
BufferTag</A>) BufferTag</A>)
<LI>MultiLevelLockTable (ctl) - <A <LI>MultiLevelLockTable (ctl) - control structure for
HREF="../../include/storage/lock.h">LOCKCTL</A> control structure for each locking method. Currently, only multi-level locking is used(<A
each locking method. Currently, only multi-level locking is used. HREF="../../include/storage/lock.h">LOCKMETHODCTL</A>).
<LI>MultiLevelLockTable (lock hash) - the <A <LI>MultiLevelLockTable (lock hash) - the <A
HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using
relation, database object ids(<A relation, database object ids(<A
@ -105,7 +106,9 @@ Each data structure is created by calling <A
HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and
the lookups are created by the lookups are created by
<A HREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A> <A HREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A>
<P>
<HR> <HR>
<P>
<CENTER> <CENTER>
<EM><BIG> <EM><BIG>
Click on an item to see more detail or Click on an item to see more detail or