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by Bruce Momjian

Queries come into the backend via data packets coming in through TCP/IP

and Unix Domain sockets.They are loaded into a string, and passed to

A query come into the backend via data packets coming in through TCP/IP

and 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-type-specific

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

structure, like

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

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

complex query.<I>Utility</I>queries are processed by a

query-type-specific function in<AHREF="../../backend/commands">

commands.</A>Complex queries, like<I>SELECT, UPDATE,</I> and

<I>DELETE</I> require much more handling.

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

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

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

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

The parser takesthe complexqueries, and creates a

The parser takesa complexquery, and creates a

<AHREF="../../include/nodes/parsenodes.h">Query</A> structure that

contains all the elements used by complex queries. Query.qual holds the

WHERE clause qualification, which is filled in by

<B>WHERE</B> clause qualification, which is filled in by

<AHREF="../../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<AHREF="../../backend/parser/parse_clause.c">

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

Certain queries, like SELECT, return columns of data. Other queries,

like INSERT and UPDATE, specify the columns modified by the query.

These column references are converted to<A

HREF="../../include/nodes/primnodes.h">Resdom</A> entries, which are

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

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

modified by the query.These column references are converted to<A

HREF="../../include/nodes/primnodes.h">Resdom</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

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

Other query elements, like aggregates(SUM()), GROUP BY, ORDER BY are

also stored in their own Query fields.

Other query elements, like aggregates(<B>SUM()</B>),<B>GROUP BY,</B>

<B>ORDER BY</B> arealso stored in their own Query fields.

The next step is for the Query to be modified by any VIEWS or RULES that

may apply to the query. This is performed by the<A

The next step is for the Query to be modified by any<B>VIEWS</B> or

<B>RULES</B> thatmay apply to the query. This is performed by the<A

HREF="../../backend/rewrite">rewrite</A> system.

The<AHREF="../../backend/optimizer">optimizer</A> takes the Query

structure, and generates an optimal

<AHREF="../..//include/nodes/plannodes.h">Plan</A> containing the

primitive operations to be performed by the executor to execute the

query. The<AHREF="../../backend/optimizer/path">path</A> module

determines the best table join order and join type of each table in the

RangeTable, using Query.qual(WHERE clause) to consider optimal index

usage.

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(<B>WHERE</B> clause) to consider optimal index usage.

The Plan is then passed to the<A

HREF="../../backend/executor">executor</A> for execution, and the result

isreturned to the client.

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.

Another area of interest is the shared memory area, containing

table data/index blocks, locks, and backend information:

<LI>ShmemIndex - contains an index of all other shared memory

structures, allowing quick lookup of other structure locations in shared

memory

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

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

Descriptors</A> - control header forshared memorybuffer block

- block oftable/index data shared by all backends

<LI><AHREF="../../include/storage/lock.h">LockTable</A>

- lock tablestructure, specifiying table, lock types, and

backends holding orwaiting on lock

<LI>LockTable (lock hash) - lookup of LockTable structures using

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

<LI>Shared Buf Lookup Table - lookup ofbuffer cache block address using

<LI><AHREF="../../include/storage/lock.h">LockTable (ctl)</A> - lock table

structure, specifiying table, lock types, and backends holding or

waiting on lock

<LI>LockTable (lock hash) - lookup of LockTable structures using relation

<LI>LockTable (xid hash) - lookup of LockTable structures using

transaction id

<LI><AHREF="../../include/storage/proc.h">Proc Header</A> - information

about each backend, including locks held/waiting, indexed by process id

Each data structure is created by calling<A

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

thelookup hashes are created by

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

thelookups are created by

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

Click on an item to see more detail or click

<AHREF="backend_dirs.html">here</a> to see the full index.

Click on an item to see more detail or

<AHREF="backend_dirs.html">click</A> to see the full index.

<IMGsrc="flow.jpg"usemap="#flowmap">

<IMGsrc="flow.jpg"usemap="#flowmap"alt="flowchart">

<MAPname="flowmap">

<AREACOORDS="290,10,450,50"HREF="backend_dirs.html#main">