<TITLE>PostgreSQLBackend Flowchart</TITLE>

<TITLE>HowPostgreSQLProcesses a Query</TITLE>

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PostgreSQLBackend Flowchart

HowPostgreSQLProcesses a Query

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by Bruce Momjian

A querycome into the backend via data packetscoming in through TCP/IP

and Unix Domain sockets. It is loaded into a string, and passed to

A querycomes to the backend via data packetsarriving through TCP/IP

or Unix Domain sockets. It is loaded into a string, and passed to

the

<AHREF="../../backend/parser">parser,</A> where the lexical scanner,

<AHREF="../../backend/parser/scan.l">scan.l,</A>

breaks the query up into tokens(words). The parser

uses

<AHREF="../../backend/parser/gram.y">gram.y</A> and the tokens to

identify the query type, and load the proper query-specific

structure,like

<AHREF="../../include/nodes/parsenodes.h">CreateStmt orSelectStmt.</A>

identify the query type, and load the proper query-specific structure,

like<AHREF="../../include/nodes/parsenodes.h">CreateStmt</A> or<A

HREF="../../include/nodes/parsenodes.h">SelectStmt.</A>

The query is then identified as a<I>Utility</I>function or a more

complexquery. A<I>Utility</I> query is processed by a

query-specific functionin<AHREF="../../backend/commands">

commands.</A> A complex query, like<CODE>SELECT, UPDATE,</CODE> and

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<AHREF="../../backend/commands"> commands.</A> A complex query, like

<CODE>SELECT, UPDATE,</CODE> and

<CODE>DELETE</CODE> requires much more handling.

The parser takes a complex query, and creates 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<AHREF="../../backend/parser/parse_clause.c">

makeRangeTable().</A> Query.rtable holds thequeries range table.

makeRangeTable().</A> Query.rtable holds thequery's range table.

Certain queries, like<CODE>SELECT,</CODE> return columns of data. Other

queries, like<CODE>INSERT</CODE> and<CODE>UPDATE,</CODE> specify the columns

<AHREF="../../backend/parser/parse_target.c">transformTargetList().</A>

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.

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

There are many other modules that support this basic functionality.

They can be accessed by clicking on the flowchart.

Another area of interest is the shared memory area, containing table

data/index blocks, locks, backend information, and lookup tables for

these structures:

Another area of interest is the shared memory area, which contains data

accessable to all backends. It has table recently used data/index

blocks, locks, backend information, and lookup tables for these

structures:

<LI>ShmemIndex - lookup shared memory addresses using structure names

<LI><AHREF="../../include/storage/buf_internals.h">Buffer

Descriptors</A> - control header for buffer cache block

<LI><AHREF="../../include/storage/buf_internals.h">BufferBlocks</A> -

Descriptor</A> - control header for buffer cache block

<LI><AHREF="../../include/storage/buf_internals.h">BufferBlock</A> -

data/index buffer cache block

<LI>SharedBuf Lookup Table - lookup of buffer cache blockaddress using

<LI>SharedBuffer Lookup Table - lookup of buffer cache blockaddresses using

table name and block number(<AHREF="../../include/storage/buf_internals.h">

BufferTag</A>)

<LI>MultiLevelLockTable (ctl) -<A

each locking method. Currently, only multi-level locking is used.

<LI>MultiLevelLockTable (ctl) -control structure for

HREF="../../include/storage/lock.h">LOCKMETHODCTL</A>).

<LI>MultiLevelLockTable (lock hash) - the<A

HREF="../../include/storage/lock.h">LOCK</A> structure, looked up using

relation, database object ids(<A

HREF="../../backend/storage/ipc/shmem.c">ShmemInitStruct(),</A> and

the lookups are created by

<AHREF="../../backend/storage/ipc/shmem.c">ShmemInitHash().</A>

Click on an item to see more detail or