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3- < TITLE > PostgreSQL Backend Flowchart </ TITLE >
3+ < TITLE > How PostgreSQL Processes a Query </ TITLE >
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66< H1 ALIGN =CENTER >
7- PostgreSQL Backend Flowchart
7+ How PostgreSQL Processes a Query
88</ H1 >
99< H2 ALIGN =CENTER >
1010by Bruce Momjian
1111</ H2 >
1212< P >
13- A query come into the backend via data packets coming in through TCP/IP
14- and Unix Domain sockets. It is loaded into a string, and passed to
13+ A query comes to the backend via data packets arriving through TCP/IP
14+ or Unix Domain sockets. It is loaded into a string, and passed to
1515the
1616< A HREF ="../../backend/parser "> parser,</ A > where the lexical scanner,
1717< A HREF ="../../backend/parser/scan.l "> scan.l,</ A >
1818breaks the query up into tokens(words). The parser
1919uses
2020< A HREF ="../../backend/parser/gram.y "> gram.y</ A > and the tokens to
21- identify the query type, and load the proper query-specific
22- structure, like
23- < A HREF ="../../include/nodes/parsenodes.h "> CreateStmt or SelectStmt.</ A >
21+ identify the query type, and load the proper query-specific structure,
22+ like < A HREF =" ../../include/nodes/parsenodes.h " > CreateStmt </ A > or < A
23+ HREF ="../../include/nodes/parsenodes.h "> SelectStmt.</ A >
2424< P >
25- The query is then identified as a < I > Utility</ I > function or a more
26- complex query. A < I > Utility</ I > query is processed by a
27- query-specific function in < A HREF ="../../backend/commands ">
28- commands. </ A > A complex query, like < CODE > SELECT, UPDATE,</ CODE > and
25+ The query is then identified as a < I > Utility</ I > query or a more complex
26+ query. A < I > Utility</ I > query is processed by a query-specific function
27+ in < A HREF ="../../backend/commands "> commands. </ A > A complex query, like
28+ < CODE > SELECT, UPDATE,</ CODE > and
2929< CODE > DELETE</ CODE > requires much more handling.
3030< P >
3131The parser takes a complex query, and creates a
@@ -37,7 +37,7 @@ <H2 ALIGN=CENTER>
3737HREF ="../../include/nodes/parsenodes.h "> RangeTableEntry,</ A > and they
3838are linked together to form the < I > range table</ I > of the query, which is
3939generated by < A HREF ="../../backend/parser/parse_clause.c ">
40- makeRangeTable().</ A > Query.rtable holds the queries range table.
40+ makeRangeTable().</ A > Query.rtable holds the query's range table.
4141< P >
4242Certain queries, like < CODE > SELECT,</ CODE > return columns of data. Other
4343queries, like < CODE > INSERT</ CODE > and < CODE > UPDATE,</ CODE > specify the columns
@@ -48,7 +48,7 @@ <H2 ALIGN=CENTER>
4848< A HREF ="../../backend/parser/parse_target.c "> transformTargetList().</ A >
4949< P >
5050Other query elements, like aggregates(< CODE > SUM()</ CODE > ), < CODE > GROUP BY,</ CODE >
51- < CODE > ORDER BY</ CODE > are also stored in their own Query fields.
51+ and < CODE > ORDER BY</ CODE > are also stored in their own Query fields.
5252< P >
5353The next step is for the Query to be modified by any < CODE > VIEWS</ CODE > or
5454< CODE > RULES</ CODE > that may apply to the query. This is performed by the < A
@@ -69,21 +69,22 @@ <H2 ALIGN=CENTER>
6969There are many other modules that support this basic functionality.
7070They can be accessed by clicking on the flowchart.
7171< P >
72- Another area of interest is the shared memory area, containing table
73- data/index blocks, locks, backend information, and lookup tables for
74- these structures:
72+ Another area of interest is the shared memory area, which contains data
73+ accessable to all backends. It has table recently used data/index
74+ blocks, locks, backend information, and lookup tables for these
75+ structures:
7576< UL >
7677< LI > ShmemIndex - lookup shared memory addresses using structure names
7778< LI > < A HREF ="../../include/storage/buf_internals.h "> Buffer
78- Descriptors </ A > - control header for buffer cache block
79- < LI > < A HREF ="../../include/storage/buf_internals.h "> Buffer Blocks </ A > -
79+ Descriptor </ A > - control header for buffer cache block
80+ < LI > < A HREF ="../../include/storage/buf_internals.h "> Buffer Block </ A > -
8081data/index buffer cache block
81- < LI > Shared Buf Lookup Table - lookup of buffer cache block address using
82+ < LI > Shared Buffer Lookup Table - lookup of buffer cache block addresses using
8283table name and block number(< A HREF ="../../include/storage/buf_internals.h ">
8384BufferTag</ A > )
84- < LI > MultiLevelLockTable (ctl) - < A
85- HREF =" ../../include/storage/lock.h " > LOCKCTL </ A > control structure for
86- each locking method. Currently, only multi-level locking is used .
85+ < LI > MultiLevelLockTable (ctl) - control structure for
86+ each locking method. Currently, only multi-level locking is used( < A
87+ HREF =" ../../include/storage/lock.h " > LOCKMETHODCTL </ A > ) .
8788< LI > MultiLevelLockTable (lock hash) - the < A
8889HREF ="../../include/storage/lock.h "> LOCK</ A > structure, looked up using
8990relation, database object ids(< A
@@ -105,7 +106,9 @@ <H2 ALIGN=CENTER>
105106HREF ="../../backend/storage/ipc/shmem.c "> ShmemInitStruct(),</ A > and
106107the lookups are created by
107108< A HREF ="../../backend/storage/ipc/shmem.c "> ShmemInitHash().</ A >
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110113< EM > < BIG >
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