本文简单介绍了PG执行SQL的流程，重点介绍了查询语句的解析（Parse）过程。
一、SQL执行流程
PG执行SQL的过程有以下几个步骤：
第一步，根据输入的SQL语句执行SQL Parse，进行词法和语法分析等，最终生成解析树；
第二步，根据解析树，执行查询逻辑/物理优化、查询重写，最终生成查询树；
第三步，根据查询树，生成执行计划；
第四步，执行器根据执行计划，执行SQL。
二、SQL解析
如前所述，PG的SQL Parse（解析）过程由函数pg_parse_query实现，在exec_simple_query函数中调用。
代码如下：
/*
* Do raw parsing (only).
*
* A list of parsetrees (RawStmt nodes) is returned, since there might be
* multiple commands in the given string.
*
* NOTE: for interactive queries, it is important to keep this routine
* separate from the analysis & rewrite stages. Analysis and rewriting
* cannot be done in an aborted transaction, since they require access to
* database tables. So, we rely on the raw parser to determine whether
* we've seen a COMMIT or ABORT command; when we are in abort state, other
* commands are not processed any further than the raw parse stage.
*/
List *
pg_parse_query(const char *query_string)
{
List *raw_parsetree_list;
TRACE_POSTGRESQL_QUERY_PARSE_START(query_string);
if (log_parser_stats)
ResetUsage();
raw_parsetree_list = raw_parser(query_string);
if (log_parser_stats)
ShowUsage("PARSER STATISTICS");
#ifdef COPY_PARSE_PLAN_TREES
/* Optional debugging check: pass raw parsetrees through copyObject() */
{
List *new_list = copyObject(raw_parsetree_list);
/* This checks both copyObject() and the equal() routines... */
if (!equal(new_list, raw_parsetree_list))
elog(WARNING, "copyObject() failed to produce an equal raw parse tree");
else
raw_parsetree_list = new_list;
}
#endif
TRACE_POSTGRESQL_QUERY_PARSE_DONE(query_string);
return raw_parsetree_list;
}
/*
* raw_parser
* Given a query in string form, do lexical and grammatical analysis.
*
* Returns a list of raw (un-analyzed) parse trees. The immediate elements
* of the list are always RawStmt nodes.
*/
List *
raw_parser(const char *str)
{
core_yyscan_t yyscanner;
base_yy_extra_type yyextra;
int yyresult;
/* initialize the flex scanner */
yyscanner = scanner_init(str, &yyextra.core_yy_extra,
ScanKeywords, NumScanKeywords);
/* base_yylex() only needs this much initialization */
yyextra.have_lookahead = false;
/* initialize the bison parser */
parser_init(&yyextra);
/* Parse! */
yyresult = base_yyparse(yyscanner);
/* Clean up (release memory) */
scanner_finish(yyscanner);
if (yyresult) /* error */
return NIL;
return yyextra.parsetree;
}
重要的数据结构:SelectStmt结构体
/* ----------------------
* Select Statement
*
* A "simple" SELECT is represented in the output of gram.y by a single
* SelectStmt node; so is a VALUES construct. A query containing set
* operators (UNION, INTERSECT, EXCEPT) is represented by a tree of SelectStmt
* nodes, in which the leaf nodes are component SELECTs and the internal nodes
* represent UNION, INTERSECT, or EXCEPT operators. Using the same node
* type for both leaf and internal nodes allows gram.y to stick ORDER BY,
* LIMIT, etc, clause values into a SELECT statement without worrying
* whether it is a simple or compound SELECT.
* ----------------------
*/
typedef enum SetOperation
{
SETOP_NONE = 0,
SETOP_UNION,
SETOP_INTERSECT,
SETOP_EXCEPT
} SetOperation;
typedef struct SelectStmt
{
NodeTag type;
/*
* These fields are used only in "leaf" SelectStmts.
*/
List *distinctClause; /* NULL, list of DISTINCT ON exprs, or
* lcons(NIL,NIL) for all (SELECT DISTINCT) */
IntoClause *intoClause; /* target for SELECT INTO */
List *targetList; /* the target list (of ResTarget) */
List *fromClause; /* the FROM clause */
Node *whereClause; /* WHERE qualification */
List *groupClause; /* GROUP BY clauses */
Node *havingClause; /* HAVING conditional-expression */
List *windowClause; /* WINDOW window_name AS (...), ... */
/*
* In a "leaf" node representing a VALUES list, the above fields are all
* null, and instead this field is set. Note that the elements of the
* sublists are just expressions, without ResTarget decoration. Also note
* that a list element can be DEFAULT (represented as a SetToDefault
* node), regardless of the context of the VALUES list. It's up to parse
* analysis to reject that where not valid.
*/
List *valuesLists; /* untransformed list of expression lists */
/*
* These fields are used in both "leaf" SelectStmts and upper-level
* SelectStmts.
*/
List *sortClause; /* sort clause (a list of SortBy's) */
Node *limitOffset; /* # of result tuples to skip */
Node *limitCount; /* # of result tuples to return */
List *lockingClause; /* FOR UPDATE (list of LockingClause's) */
WithClause *withClause; /* WITH clause */
/*
* These fields are used only in upper-level SelectStmts.
*/
SetOperation op; /* type of set op */
bool all; /* ALL specified? */
struct SelectStmt *larg; /* left child */
struct SelectStmt *rarg; /* right child */
/* Eventually add fields for CORRESPONDING spec here */
} SelectStmt;
重要的结构体:Value
/*----------------------
* Value node
*
* The same Value struct is used for five node types: T_Integer,
* T_Float, T_String, T_BitString, T_Null.
*
* Integral values are actually represented by a machine integer,
* but both floats and strings are represented as strings.
* Using T_Float as the node type simply indicates that
* the contents of the string look like a valid numeric literal.
*
* (Before Postgres 7.0, we used a double to represent T_Float,
* but that creates loss-of-precision problems when the value is
* ultimately destined to be converted to NUMERIC. Since Value nodes
* are only used in the parsing process, not for runtime data, it's
* better to use the more general representation.)
*
* Note that an integer-looking string will get lexed as T_Float if
* the value is too large to fit in an 'int'.
*
* Nulls, of course, don't need the value part at all.
*----------------------
*/
typedef struct Value
{
NodeTag type; /* tag appropriately (eg. T_String) */
union ValUnion
{
int ival; /* machine integer */
char *str; /* string */
} val;
} Value;
#define intVal(v) (((Value *)(v))->val.ival)
#define floatVal(v) atof(((Value *)(v))->val.str)
#define strVal(v) (((Value *)(v))->val.str)
实现过程本节暂时搁置，先看过程执行的结果，函数pg_parse_query返回的结果是链表List，其中的元素是RawStmt，具体的结构需根据NodeTag确定（这样的做法类似于Java/C++的多态）。
测试数据
testdb=# -- 单位信息
testdb=# drop table if exists t_dwxx;
ues('Y有限公司','1002','北京市海淀区');
insert into t_dwxx(dwmc,dwbh,dwdz) values('Z有限公司','1003','广西南宁市五象区');
NOTICE: table "t_dwxx" does not exist, skipping
DROP TABLE
testdb=# create table t_dwxx(dwmc varchar(100),dwbh varchar(10),dwdz varchar(100));
CREATE TABLE
testdb=#
testdb=# insert into t_dwxx(dwmc,dwbh,dwdz) values('X有限公司','1001','广东省广州市荔湾区');
INSERT 0 1
testdb=# insert into t_dwxx(dwmc,dwbh,dwdz) values('Y有限公司','1002','北京市海淀区');
INSERT 0 1
testdb=# insert into t_dwxx(dwmc,dwbh,dwdz) values('Z有限公司','1003','广西南宁市五象区');
INSERT 0 1
testdb=# -- 个人信息
testdb=# drop table if exists t_grxx;
NOTICE: table "t_grxx" does not exist, skipping
DROP TABLE
testdb=# create table t_grxx(dwbh varchar(10),grbh varchar(10),xm varchar(20),nl int);
CREATE TABLE
insert into t_grxx(dwbh,grbh,xm,nl) values('1002','903','王五',43);
testdb=#
testdb=# insert into t_grxx(dwbh,grbh,xm,nl) values('1001','901','张三',23);
INSERT 0 1
testdb=# insert into t_grxx(dwbh,grbh,xm,nl) values('1002','902','李四',33);
INSERT 0 1
testdb=# insert into t_grxx(dwbh,grbh,xm,nl) values('1002','903','王五',43);
INSERT 0 1
testdb=# -- 个人缴费信息
testdb=# drop table if exists t_jfxx;
NOTICE: table "t_jfxx" does not exist, skipping
DROP TABLE
testdb=# create table t_jfxx(grbh varchar(10),ny varchar(10),je float);
CREATE TABLE
testdb=#
testdb=# insert into t_jfxx(grbh,ny,je) values('901','201801',401.30);
insert into t_jfxx(grbh,ny,je) values('901','201802',401.30);
insert into t_jfxx(grbh,ny,je) values('901','201803',401.30);
insert into t_jfxx(grbh,ny,je) values('902','201801',513.30);
insert into t_jfxx(grbh,ny,je) values('902','201802',513.30);
insert into t_jfxx(grbh,ny,je) values('902','201804',513.30);
insert into t_jfxx(grbh,ny,je) values('903','201801',372.22);
insert into t_jfxx(grbh,ny,je) values('903','201804',372.22);
testdb=# insert into t_jfxx(grbh,ny,je) values('901','201801',401.30);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('901','201802',401.30);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('901','201803',401.30);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('902','201801',513.10);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('902','201802',513.30);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('902','201804',513.30);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('903','201801',372.22);
INSERT 0 1
testdb=# insert into t_jfxx(grbh,ny,je) values('903','201804',372.22);
INSERT 0 1
testdb=# -- 获取pid
testdb=# select pg_backend_pid();
pg_backend_pid
----------------
1560
(1 row)
-- 用于测试的查询语句
testdb=# select t_dwxx.dwmc,t_grxx.grbh,t_grxx.xm,t_jfxx.ny,t_jfxx.je
testdb-# from t_dwxx,t_grxx,t_jfxx
testdb-# where t_dwxx.dwbh = t_grxx.dwbh
testdb-# and t_grxx.grbh = t_jfxx.grbh
testdb-# and t_dwxx.dwbh IN ('1001','1002')
testdb-# order by t_grxx.grbh
testdb-# limit 8;
dwmc | grbh | xm | ny | je
-----------+------+------+--------+--------
X有限公司 | 901 | 张三 | 201801 | 401.3
X有限公司 | 901 | 张三 | 201802 | 401.3
X有限公司 | 901 | 张三 | 201803 | 401.3
Y有限公司 | 902 | 李四 | 201801 | 513.1
Y有限公司 | 902 | 李四 | 201802 | 513.3
Y有限公司 | 902 | 李四 | 201804 | 513.3
Y有限公司 | 903 | 王五 | 201801 | 372.22
Y有限公司 | 903 | 王五 | 201804 | 372.22
(8 rows)
结果分析
[xdb@localhost ~]$ gdb -p 1560
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-100.el7
Copyright (C) 2013 Free Software Foundation, Inc.
...
(gdb) b pg_parse_query
Breakpoint 1 at 0x84c6c9: file postgres.c, line 615.
(gdb) c
Continuing.
Breakpoint 1, pg_parse_query (
query_string=0x1a46ef0 "select t_dwxx.dwmc,t_grxx.grbh,t_grxx.xm,t_jfxx.ny,t_jfxx.je\nfrom t_dwxx inner join t_grxx on t_dwxx.dwbh = t_grxx.dwbh\ninner join t_jfxx on t_grxx.grbh = t_jfxx.grbh\nwhere t_dwxx.dwbh IN ('1001','100"...) at postgres.c:615
615 if (log_parser_stats)
(gdb) n
618 raw_parsetree_list = raw_parser(query_string);
(gdb)
620 if (log_parser_stats)
(gdb)
638 return raw_parsetree_list;
(gdb) p *(RawStmt *)(raw_parsetree_list->head.data->ptr_value)
$7 = {type = T_RawStmt, stmt = 0x1a48c00, stmt_location = 0, stmt_len = 232}
(gdb) p *((RawStmt *)(raw_parsetree_list->head.data->ptr_value))->stmt
$8 = {type = T_SelectStmt}
#转换为实际类型SelectStmt
(gdb) p *(SelectStmt *)((RawStmt *)(raw_parsetree_list->head.data->ptr_value))->stmt
$16 = {type = T_SelectStmt, distinctClause = 0x0, intoClause = 0x0, targetList = 0x1a47b18,
fromClause = 0x1a48900, whereClause = 0x1a48b40, groupClause = 0x0, havingClause = 0x0, windowClause = 0x0,
valuesLists = 0x0, sortClause = 0x1afd858, limitOffset = 0x0, limitCount = 0x1afd888, lockingClause = 0x0,
withClause = 0x0, op = SETOP_NONE, all = false, larg = 0x0, rarg = 0x0}
#设置临时变量
(gdb) set $stmt=(SelectStmt *)((RawStmt *)(raw_parsetree_list->head.data->ptr_value))->stmt
#查看结构体中的各个变量
#------------------->targetList
(gdb) p *($stmt->targetList)
$28 = {type = T_List, length = 5, head = 0x1a47af8, tail = 0x1a48128}
#targetList有5个元素,分别对应t_dwxx.dwmc,t_grxx.grbh,t_grxx.xm,t_jfxx.ny,t_jfxx.je
#先看第1个元素
(gdb) set $restarget=(ResTarget *)($stmt->targetList->head.data->ptr_value)
(gdb) p *$restarget->val
$25 = {type = T_ColumnRef}
(gdb) p *(ColumnRef *)$restarget->val
$26 = {type = T_ColumnRef, fields = 0x1a47a08, location = 7}
(gdb) p *((ColumnRef *)$restarget->val)->fields
$27 = {type = T_List, length = 2, head = 0x1a47a88, tail = 0x1a479e8}
(gdb) p *(Node *)(((ColumnRef *)$restarget->val)->fields)->head.data->ptr_value
$32 = {type = T_String}
#fields链表的第1个元素是数据表,第2个元素是数据列
(gdb) p *(Value *)(((ColumnRef *)$restarget->val)->fields)->head.data->ptr_value
$37 = {type = T_String, val = {ival = 27556248, str = 0x1a47998 "t_dwxx"}}
(gdb) p *(Value *)(((ColumnRef *)$restarget->val)->fields)->tail.data->ptr_value
$38 = {type = T_String, val = {ival = 27556272, str = 0x1a479b0 "dwmc"}}
#其他类似
#------------------->fromClause
(gdb) p *(Node *)($stmt->fromClause->head.data->ptr_value)
$41 = {type = T_JoinExpr}
(gdb) set $fromclause=(JoinExpr *)($stmt->fromClause->head.data->ptr_value)
(gdb) p *$fromclause
$42 = {type = T_JoinExpr, jointype = JOIN_INNER, isNatural = false, larg = 0x1a484f8, rarg = 0x1a48560,
usingClause = 0x0, quals = 0x1a487d0, alias = 0x0, rtindex = 0}
#------------------->whereClause
(gdb) p *(Node *)($stmt->whereClause)
$44 = {type = T_A_Expr}
(gdb) p *(FromExpr *)($stmt->whereClause)
$46 = {type = T_A_Expr, fromlist = 0x1a48bd0, quals = 0x1a489d0}
#------------------->sortClause
(gdb) p *(Node *)($stmt->sortClause->head.data->ptr_value)
$48 = {type = T_SortBy}
(gdb) p *(SortBy *)($stmt->sortClause->head.data->ptr_value)
$49 = {type = T_SortBy, node = 0x1a48db0, sortby_dir = SORTBY_DEFAULT, sortby_nulls = SORTBY_NULLS_DEFAULT,
useOp = 0x0, location = -1}
#------------------->limitCount
(gdb) p *(Node *)($stmt->limitCount)
$50 = {type = T_A_Const}
(gdb) p *(Const *)($stmt->limitCount)
$51 = {xpr = {type = T_A_Const}, consttype = 0, consttypmod = 216, constcollid = 0, constlen = 8,
constvalue = 231, constisnull = 16, constbyval = false, location = 0}
以上为简单的数据结构介绍，下一节将详细解析parseTree的Tree结构。
三、小结
1、SQL执行流程：简单介绍了SQL的执行流程，简单分为解析、查询优化、执行计划生成和执行这四步；
2、SQL解析：解析执行后，结果存储在解析树中，分为distinctClause、intoClause、targetList等多个部分。
