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, skippingDROP TABLEtestdb=# create table t_dwxx(dwmc varchar(100),dwbh varchar(10),dwdz varchar(100));CREATE TABLEtestdb=# 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 1testdb=# insert into t_dwxx(dwmc,dwbh,dwdz) values('Y有限公司','1002','北京市海淀区');INSERT 0 1testdb=# insert into t_dwxx(dwmc,dwbh,dwdz) values('Z有限公司','1003','广西南宁市五象区');INSERT 0 1testdb=# -- 个人信息testdb=# drop table if exists t_grxx;NOTICE:  table "t_grxx" does not exist, skippingDROP TABLEtestdb=# create table t_grxx(dwbh varchar(10),grbh varchar(10),xm varchar(20),nl int);CREATE TABLEinsert 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 1testdb=# insert into t_grxx(dwbh,grbh,xm,nl) values('1002','902','李四',33);INSERT 0 1testdb=# insert into t_grxx(dwbh,grbh,xm,nl) values('1002','903','王五',43);INSERT 0 1testdb=# -- 个人缴费信息testdb=# drop table if exists t_jfxx;NOTICE:  table "t_jfxx" does not exist, skippingDROP TABLEtestdb=# create table t_jfxx(grbh varchar(10),ny varchar(10),je float);CREATE TABLEtestdb=# 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 1testdb=# insert into t_jfxx(grbh,ny,je) values('901','201802',401.30);INSERT 0 1testdb=# insert into t_jfxx(grbh,ny,je) values('901','201803',401.30);INSERT 0 1testdb=# insert into t_jfxx(grbh,ny,je) values('902','201801',513.10);INSERT 0 1testdb=# insert into t_jfxx(grbh,ny,je) values('902','201802',513.30);INSERT 0 1testdb=# insert into t_jfxx(grbh,ny,je) values('902','201804',513.30);INSERT 0 1testdb=# insert into t_jfxx(grbh,ny,je) values('903','201801',372.22);INSERT 0 1testdb=# insert into t_jfxx(grbh,ny,je) values('903','201804',372.22);INSERT 0 1testdb=# -- 获取pidtestdb=# 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.jetestdb-# from t_dwxx,t_grxx,t_jfxxtestdb-# where t_dwxx.dwbh = t_grxx.dwbh testdb-# and t_grxx.grbh = t_jfxx.grbhtestdb-# and t_dwxx.dwbh IN ('1001','1002')testdb-# order by t_grxx.grbhtestdb-# 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 1560GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-100.el7Copyright (C) 2013 Free Software Foundation, Inc....(gdb) b pg_parse_queryBreakpoint 1 at 0x84c6c9: file postgres.c, line 615.(gdb) cContinuing.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:615615     if (log_parser_stats)(gdb) n618     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。