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148 lines
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HTML
148 lines
6.7 KiB
HTML
<HTML>
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<HEAD>
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<TITLE>PostgreSQL Backend Flowchart</TITLE>
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</HEAD>
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<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
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<H1 ALIGN=CENTER>
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PostgreSQL Backend Flowchart
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</H1>
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<H2 ALIGN=CENTER>
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by Bruce Momjian
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</H2>
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<P>
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A query come into the backend via data packets coming in through TCP/IP
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and Unix Domain sockets. It is loaded into a string, and passed to
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the
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<A HREF="../../backend/parser">parser,</A> where the lexical scanner,
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<A HREF="../../backend/parser/scan.l">scan.l,</A>
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breaks the query up into tokens(words). The parser
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uses
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<A HREF="../../backend/parser/gram.y">gram.y</A> and the tokens to
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identify the query type, and load the proper query-specific
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structure, like
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<A HREF="../../include/nodes/parsenodes.h">CreateStmt or SelectStmt.</A>
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<P>
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The query is then identified as a <I>Utility</I> function or a more
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complex query. A <I>Utility</I> query is processed by a
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query-specific function in <A HREF="../../backend/commands">
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commands.</A> A complex query, like <CODE>SELECT, UPDATE,</CODE> and
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<CODE>DELETE</CODE> requires much more handling.
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<P>
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The parser takes a complex query, and creates a
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<A HREF="../../include/nodes/parsenodes.h">Query</A> structure that
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contains all the elements used by complex queries. Query.qual holds the
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<CODE>WHERE</CODE> clause qualification, which is filled in by
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<A HREF="../../backend/parser/parse_clause.c">transformWhereClause().</A>
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Each table referenced in the query is represented by a <A
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HREF="../../include/nodes/parsenodes.h"> RangeTableEntry,</A> and they
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are linked together to form the <I>range table</I> of the query, which is
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generated by <A HREF="../../backend/parser/parse_clause.c">
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makeRangeTable().</A> Query.rtable holds the queries range table.
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<P>
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Certain queries, like <CODE>SELECT,</CODE> return columns of data. Other
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queries, like <CODE>INSERT</CODE> and <CODE>UPDATE,</CODE> specify the columns
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modified by the query. These column references are converted to <A
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HREF="../../include/nodes/primnodes.h">Resdom</A> entries, which are
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linked together to make up the <I>target list</I> of the query. The
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target list is stored in Query.targetList, which is generated by
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<A HREF="../../backend/parser/parse_target.c">transformTargetList().</A>
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<P>
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Other query elements, like aggregates(<CODE>SUM()</CODE>), <CODE>GROUP BY,</CODE>
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<CODE>ORDER BY</CODE> are also stored in their own Query fields.
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<P>
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The next step is for the Query to be modified by any <CODE>VIEWS</CODE> or
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<CODE>RULES</CODE> that may apply to the query. This is performed by the <A
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HREF="../../backend/rewrite">rewrite</A> system.
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<P>
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The <A HREF="../../backend/optimizer">optimizer</A> takes the Query
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structure and generates an optimal <A
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HREF="../..//include/nodes/plannodes.h">Plan,</A> which contains the
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operations to be performed to execute the query. The <A
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HREF="../../backend/optimizer/path">path</A> module determines the best
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table join order and join type of each table in the RangeTable, using
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Query.qual(<CODE>WHERE</CODE> clause) to consider optimal index usage.
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<P>
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The Plan is then passed to the <A
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HREF="../../backend/executor">executor</A> for execution, and the result
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returned to the client.
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<P>
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There are many other modules that support this basic functionality.
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They can be accessed by clicking on the flowchart.
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<P>
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Another area of interest is the shared memory area, containing table
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data/index blocks, locks, backend information, and lookup tables for
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these structures:
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<UL>
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<LI>ShmemIndex - lookup shared memory addresses using structure names
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<LI><A HREF="../../include/storage/buf_internals.h">Buffer
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Descriptors</A> - control header for buffer cache block
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<LI><A HREF="../../include/storage/buf_internals.h">Buffer Blocks</A> -
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data/index buffer cache block
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<LI>Shared Buf Lookup Table - lookup of buffer cache block address using
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table name and block number(<A HREF="../../include/storage/buf_internals.h">
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BufferTag</A>)
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<LI>MultiLevelLockTable (ctl) - <A
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HREF="../../include/storage/lock.h">LOCKCTL</A> control structure for
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each locking method. Currently, only multi-level locking is used.
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<LI>MultiLevelLockTable (lock hash) - the <A
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HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using
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relation, database object ids(<A
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HREF="../../include/storage/lock.h">LOCKTAG)</A>. The lock table structure contains the
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lock modes(read, write) and circular linked list of backends (<A
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HREF="../../include/storage/proc.h">PROC</A> structure pointers) waiting
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on the lock.
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<LI>MultiLevelLockTable (xid hash) - lookup of LOCK structure address
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using transaction id, LOCK address. It is used to quickly check if the
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current transaction already has any locks on a table, rather than having
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to search through all the held locks. It also stores the modes
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(read/write) of the locks held by the current transaction. The returned
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<A HREF="../../include/storage/lock.h">XIDLookupEnt</A> structure also
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contains a pointer to the backend's PROC.lockQueue.
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<LI><A HREF="../../include/storage/proc.h">Proc Header</A> - information
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about each backend, including locks held/waiting, indexed by process id
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</UL>
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Each data structure is created by calling <A
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HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and
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the lookups are created by
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<A HREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A>
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<HR>
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<CENTER>
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<EM><BIG>
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Click on an item to see more detail or
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<A HREF="backend_dirs.html">click</A> to see the full index.
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</BIG></EM>
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<BR>
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<BR>
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<IMG src="flow.jpg" usemap="#flowmap" alt="flowchart">
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</CENTER>
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<MAP name="flowmap">
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<AREA COORDS="290,10,450,50" HREF="backend_dirs.html#main">
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<AREA COORDS="550,10,710,50" HREF="backend_dirs.html#bootstrap">
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<AREA COORDS="290,90,450,130," HREF="backend_dirs.html#postmaster">
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<AREA COORDS="550,90,710,130," HREF="backend_dirs.html#libpq">
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<AREA COORDS="290,170,450,210" HREF="backend_dirs.html#tcop">
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<AREA COORDS="550,170,710,210" HREF="backend_dirs.html#tcop">
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<AREA COORDS="290,270,450,310" HREF="backend_dirs.html#parser">
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<AREA COORDS="290,350,450,390" HREF="backend_dirs.html#tcop">
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<AREA COORDS="290,430,450,470" HREF="backend_dirs.html#optimizer">
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<AREA COORDS="290,510,450,550" HREF="backend_dirs.html#optimizer/plan">
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<AREA COORDS="290,570,450,630" HREF="backend_dirs.html#executor">
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<AREA COORDS="550,350,710,390" HREF="backend_dirs.html#commands">
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<AREA COORDS="10,330,170,370" HREF="backend_dirs.html#access">
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<AREA COORDS="10,390,170,430" HREF="backend_dirs.html#catalog">
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<AREA COORDS="10,450,170,490" HREF="backend_dirs.html#utils">
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<AREA COORDS="10,510,170,550" HREF="backend_dirs.html#nodes">
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<AREA COORDS="10,570,170,610" HREF="backend_dirs.html#storage">
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</MAP>
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<BR>
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<HR SIZE="2" NOSHADE>
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<SMALL>
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<ADDRESS>
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Maintainer: Bruce Momjian (<A
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HREF="mailto:maillist@candle.pha.pa.us">maillist@candle.pha.pa.us</A>)<BR>
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Last updated: Tue Dec 9 17:56:08 EST 1997
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</ADDRESS>
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</SMALL>
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</BODY>
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</HTML>
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