PostgreSQL兼容Oracle語法研究——外關(guān)聯(lián)

一刊愚、背景

在oracle遷移postgres中，為了盡量減少對原有系統(tǒng)的改造踩验，除了遷移數(shù)據(jù)之外鸥诽，postgres對oracle的SQL語法兼容也是必不可少的一部分。

本文主要研究postgres對oracle 的外關(guān)聯(lián)兼容性晰甚。

oracle連接的語法分為2類：
1衙传、oracle專用語法（傳統(tǒng)語法，主要是指sql:89語法厕九，被廣泛支持,是oracle專門支持的語法）
2蓖捶、ANSI SQL:99（標準語法，oracle在8i后引入了sql99的語法扁远，各廠商據(jù)此發(fā)展自己的sql：T-SQL,PL/SQL）

Oracle為了適應(yīng)大多數(shù)人的使用俊鱼，既支持了oracle專用語法，也支持了ANSI SQL:99語法畅买，所以oracle 9i實現(xiàn)多表之間的連接既可以用oracle專用語法并闲，也可以用ANSISQL:99語法。兩種語法沒有效率上的提高谷羞。
但對一些古董級別的oracle使用者帝火，沿用oracle專用語法溜徙，可以保證自己的SQL兼容oracle 8i 之前的數(shù)據(jù)庫系統(tǒng)。

注：現(xiàn)有的業(yè)務(wù)系統(tǒng)源碼犀填，很多都有這部分古董級別的蹤跡蠢壹。

二、準備測試數(shù)據(jù)

CREATE TABLE t_A (
id   int,
code   int,
name  VARCHAR(10)
);
 
CREATE TABLE t_B (
id   int,
code   int,
name  VARCHAR(10)
);
 
INSERT INTO t_A(id,code,name) VALUES(1,2,'A');
INSERT INTO t_A(id,code,name) VALUES(2,1,'B');
INSERT INTO t_A(id,code,name) VALUES(3,5,'C');
INSERT INTO t_A(id,code,name) VALUES(4,6,'D');
INSERT INTO t_A(id,code,name) VALUES(5,7,'E');
 
INSERT INTO t_B(id,code,name) VALUES(1,3,'AA');
INSERT INTO t_B(id,code,name) VALUES(1,4,'BB');
INSERT INTO t_B(id,code,name) VALUES(2,1,'CC');
INSERT INTO t_B(id,code,name) VALUES(1,2,'DD');
INSERT INTO t_B(id,code,name) VALUES(7,5,'GG');

三九巡、語法分析過程

oracle專用標準的關(guān)聯(lián)查詢图贸，以兩個關(guān)聯(lián)鍵值的左關(guān)聯(lián)SQL為例。

select * 
from t_a a,t_b b 
where a.id=b.id(+) and a.code=b.code(+) 
    and a.id!=8;

轉(zhuǎn)換為ANSI標準的left /right/full join 的寫法如下：

select * 
from t_a a 
left join t_b b on a.id=b.id and a.code=b.code
where a.id!=8;

斷點分析外關(guān)聯(lián)語法樹：

/*連接數(shù)據(jù)庫冕广，并獲取當(dāng)前連接的pid疏日。*/
[appusr@postgre ~]$ psql -h 127.0.0.1 -d postgres
psql (11beta2)
Type "help" for help.

postgres=# select pg_backend_pid();                                                       
 pg_backend_pid 
----------------
          10473
(1 row)

/*切換到gdb*/
[appusr@postgre ~]$ gdb
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-110.el7
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
(gdb) attach 10473
Attaching to process 10473
...
/* 設(shè)置斷點 */
(gdb) b exec_simple_query 
Breakpoint 1 at 0x8c35db: file postgres.c, line 893.
(gdb) c
Continuing.

/*切換到psql，執(zhí)行查詢*/
postgres=# select * from t_a a 
left join t_b b on a.id=b.id and a.code=b.code
where a.id!=8;

/*切換到gdb撒汉，分析語法樹*/
Breakpoint 1, exec_simple_query (query_string=0x2574968 "select * from t_a a \nleft join t_b b on a.id=b.id and a.code=b.code\nwhere a.id!=8;") at postgres.c:893
893             CommandDest dest = whereToSendOutput;
(gdb) n
897             bool            save_log_statement_stats = log_statement_stats;
(gdb) 
898             bool            was_logged = false;
(gdb) 
905             debug_query_string = query_string;
(gdb) 
907             pgstat_report_activity(STATE_RUNNING, query_string);
(gdb) 
909             TRACE_POSTGRESQL_QUERY_START(query_string);
(gdb) 
915             if (save_log_statement_stats)
(gdb) 
925             start_xact_command();
(gdb) 
933             drop_unnamed_stmt();
(gdb) 
938             oldcontext = MemoryContextSwitchTo(MessageContext);
(gdb) 
944             parsetree_list = pg_parse_query(query_string); /* 一個字符串內(nèi)可能含有多個SQL沟优，所以語法分析的結(jié)果是列表結(jié)構(gòu) */
(gdb) 
947             if (check_log_statement(parsetree_list))
(gdb) 
949                     ereport(LOG,
(gdb) 
953                     was_logged = true;
(gdb) 
959             MemoryContextSwitchTo(oldcontext);
(gdb) 
969             use_implicit_block = (list_length(parsetree_list) > 1);
(gdb) 
974             foreach(parsetree_item, parsetree_list)
(gdb) 
976                     RawStmt    *parsetree = lfirst_node(RawStmt, parsetree_item); /* 一個SQL的語法樹 */
(gdb) 
977                     bool            snapshot_set = false;
(gdb) p (*parsetree)
$1 = {type = T_RawStmt, stmt = 0x25761b0, stmt_location = 0, stmt_len = 81}
(gdb) p *(parsetree->stmt)
$2 = {type = T_SelectStmt}
(gdb) p *((SelectStmt*)parsetree->stmt)
$3 = {type = T_SelectStmt, distinctClause = 0x0, intoClause = 0x0, targetList = 0x25754f8, fromClause = 0x2575f10, whereClause = 0x25760d0, groupClause = 0x0, havingClause = 0x0, windowClause = 0x0, 
  valuesLists = 0x0, sortClause = 0x0, limitOffset = 0x0, limitCount = 0x0, lockingClause = 0x0, withClause = 0x0, op = SETOP_NONE, all = false, larg = 0x0, rarg = 0x0}

/* 分析targetList結(jié)構(gòu) */
(gdb) p *((SelectStmt*)parsetree->stmt)->targetList
$4 = {type = T_List, length = 1, head = 0x25754d0, tail = 0x25754d0}
(gdb) p *((SelectStmt*)parsetree->stmt)->targetList->head
$5 = {data = {ptr_value = 0x2575478, int_value = 39277688, oid_value = 39277688}, next = 0x0}
(gdb) p *((Node*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)
$6 = {type = T_ResTarget}
(gdb) p *((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)
$7 = {type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x25753c0, location = 7}
(gdb) p *((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val
$8 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)
$9 = {type = T_ColumnRef, fields = 0x2575440, location = 7}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields
$10 = {type = T_List, length = 1, head = 0x2575418, tail = 0x2575418}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields->head
$11 = {data = {ptr_value = 0x25753f8, int_value = 39277560, oid_value = 39277560}, next = 0x0}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb) p *((Node*)((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)
$12 = {type = T_A_Star}
(gdb) p *((A_Star*)((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)
$13 = {type = T_A_Star}  /* 標識 '*' */

/* 分析fromClause結(jié)構(gòu) */
(gdb) p *((SelectStmt*)parsetree->stmt)->fromClause
$14 = {type = T_List, length = 1, head = 0x2575ee8, tail = 0x2575ee8}
(gdb) p *((Node*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)
$15 = {type = T_JoinExpr}
(gdb) p *((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)
$16 = {type = T_JoinExpr, jointype = JOIN_LEFT, isNatural = false, larg = 0x2575570, rarg = 0x2575640, usingClause = 0x0, quals = 0x2575e58, alias = 0x0, rtindex = 0}
(gdb) p *((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->larg
$17 = {type = T_RangeVar}
(gdb) p *((RangeVar*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->larg)
$18 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x2575530 "t_a", inh = true, relpersistence = 112 'p', alias = 0x25755c8, location = 14}
(gdb) p *((RangeVar*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->larg)->alias
$19 = {type = T_Alias, aliasname = 0x2575550 "a", colnames = 0x0}
(gdb) p *((RangeVar*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->rarg)
$20 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x2575600 "t_b", inh = true, relpersistence = 112 'p', alias = 0x2575698, location = 31}
(gdb) p *((RangeVar*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->rarg)->alias
$21 = {type = T_Alias, aliasname = 0x2575620 "b", colnames = 0x0}
 
/*分析join on條件*/
(gdb) p *((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals
$22 = {type = T_BoolExpr}
(gdb) p *((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)
$23 = {xpr = {type = T_BoolExpr}, boolop = AND_EXPR, args = 0x2575df8, location = 50}
(gdb) p *((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->agrs
There is no member named agrs.
(gdb) p *((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args
$24 = {type = T_List, length = 2, head = 0x2575e30, tail = 0x2575dd0} /*注意，長度為2神凑，列表中有兩個節(jié)點*/
(gdb) p *((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value
Attempt to dereference a generic pointer.

/* 分析join on第一個條件净神，args的第一個元素:a.id=b.id */
(gdb) p *((Node*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)
$25 = {type = T_A_Expr}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)
$26 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x2575a18, lexpr = 0x2575798, rexpr = 0x25758e8, location = 44}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->name
$27 = {type = T_List, length = 1, head = 0x25759f0, tail = 0x25759f0}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->name->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb) p *((Node*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->name->head->data->ptr_value)
$28 = {type = T_String}
(gdb) p *((Value*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->name->head->data->ptr_value)
$29 = {type = T_String, val = {ival = 12092910, str = 0xb885ee "="}}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr
$30 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr)
$31 = {type = T_ColumnRef, fields = 0x2575760, location = 40}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr)->fields
$32 = {type = T_List, length = 2, head = 0x25757f8, tail = 0x2575738}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$33 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$34 = {type = T_String, val = {ival = 39278288, str = 0x25756d0 "a"}}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$35 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$36 = {type = T_String, val = {ival = 39278320, str = 0x25756f0 "id"}}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr
$37 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr)
$38 = {type = T_ColumnRef, fields = 0x25758b0, location = 45}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr)->fields
$39 = {type = T_List, length = 2, head = 0x2575948, tail = 0x2575888}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr)->fields->head->data->ptr_value)
$40 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr)->fields->head->data->ptr_value)
$41 = {type = T_String, val = {ival = 39278624, str = 0x2575820 "b"}}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr)->fields->head->next->data->ptr_value)
$42 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->data->ptr_value)->rexpr)->fields->head->next->data->ptr_value)
$43 = {type = T_String, val = {ival = 39278656, str = 0x2575840 "id"}}

/* 分析join on第二個條件，args的第二個元素:a.code=b.code */
(gdb) p *((Node*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)
$44 = {type = T_A_Expr}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)
$45 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x2575d98, lexpr = 0x2575b18, rexpr = 0x2575c68, location = 60}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->name
$46 = {type = T_List, length = 1, head = 0x2575d70, tail = 0x2575d70}
(gdb) p *((Node*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->name->head->data->ptr_value)
$47 = {type = T_String}
(gdb) p *((Value*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->name->head->data->ptr_value)
$48 = {type = T_String, val = {ival = 12092910, str = 0xb885ee "="}}
(gdb) p *((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->lexpr
$49 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->lexpr)
$50 = {type = T_ColumnRef, fields = 0x2575ae0, location = 54}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->lexpr)->fields
$51 = {type = T_List, length = 2, head = 0x2575b78, tail = 0x2575ab8}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$52 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$53 = {type = T_String, val = {ival = 39279184, str = 0x2575a50 "a"}}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$54 = {type = T_String, val = {ival = 39279216, str = 0x2575a70 "code"}}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->rexpr)
$55 = {type = T_ColumnRef, fields = 0x2575c30, location = 61}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->rexpr)->fields
$56 = {type = T_List, length = 2, head = 0x2575cc8, tail = 0x2575c08}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->rexpr)->fields->head->data->ptr_value)
$57 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->rexpr)->fields->head->data->ptr_value)
$58 = {type = T_String, val = {ival = 39279520, str = 0x2575ba0 "b"}}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((JoinExpr*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->quals)->args->head->next->data->ptr_value)->rexpr)->fields->head->next->data->ptr_value)
$59 = {type = T_String, val = {ival = 39279552, str = 0x2575bc0 "code"}}

/* 分析where條件部分:a.id=8*/
(gdb) p *((SelectStmt*)parsetree->stmt)->whereClause
$60 = {type = T_A_Expr}
(gdb) p *((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)
$61 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x2576178, lexpr = 0x2576010, rexpr = 0x2576098, location = 78}
(gdb) p *((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->name
$62 = {type = T_List, length = 1, head = 0x2576150, tail = 0x2576150}
(gdb) p *((Node*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->name->head->data->ptr_value)
$63 = {type = T_String}
(gdb) p *((Value*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->name->head->data->ptr_value)
$64 = {type = T_String, val = {ival = 12092918, str = 0xb885f6 "<>"}}
(gdb) p *((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->lexpr
$65 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->lexpr)
$66 = {type = T_ColumnRef, fields = 0x2575fd8, location = 74}
(gdb) p *((ColumnRef*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->lexpr)->fields
$67 = {type = T_List, length = 2, head = 0x2576070, tail = 0x2575fb0}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->lexpr)->fields->head->data->ptr_value)
$68 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->lexpr)->fields->head->data->ptr_value)
$69 = {type = T_String, val = {ival = 39280456, str = 0x2575f48 "a"}}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->lexpr)->fields->head->next->data->ptr_value)
$70 = {type = T_String, val = {ival = 39280488, str = 0x2575f68 "id"}}
(gdb) p *((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->rexpr
$71 = {type = T_A_Const}
(gdb) p *((A_Const*)((A_Expr*)((SelectStmt*)parsetree->stmt)->whereClause)->rexpr)
$72 = {type = T_A_Const, val = {type = T_Integer, val = {ival = 8, str = 0x8 <Address 0x8 out of bounds>}}, location = 80}
(gdb)

parsetree的結(jié)構(gòu)：


外關(guān)聯(lián)語法樹

因為現(xiàn)在還沒有改造溉委，無法識別oracle專用語法

select * 
from t_a a,t_b b 
where a.id=b.id(+) and a.code=b.code(+) 
    and a.id!=8;

先分析對應(yīng)的內(nèi)關(guān)聯(lián)SQL查詢（刪除"(+)"字符串之后的）：

select * 
from t_a a,t_b b 
where a.id=b.id and a.code=b.code
    and a.id!=8;

/*切換到psql鹃唯，執(zhí)行查詢*/
postgres=# select * 
from t_a a,t_b b 
where a.id=b.id and a.code=b.code
    and a.id!=8;

/*切換到gdb，分析語法樹*/
Breakpoint 1, exec_simple_query (query_string=0x2574968 "select * \nfrom t_a a,t_b b \nwhere a.id=b.id and a.code=b.code and a.id!=8;") at postgres.c:893
893             CommandDest dest = whereToSendOutput;
(gdb) bt
#0  exec_simple_query (query_string=0x2574968 "select * \nfrom t_a a,t_b b \nwhere a.id=b.id and a.code=b.code and a.id!=8;") at postgres.c:893
#1  0x00000000008c7d23 in PostgresMain (argc=1, argv=0x25a0890, dbname=0x25a06f0 "postgres", username=0x25a06d0 "appusr") at postgres.c:4153
#2  0x000000000082405c in BackendRun (port=0x25966a0) at postmaster.c:4361
#3  0x00000000008237c0 in BackendStartup (port=0x25966a0) at postmaster.c:4033
#4  0x000000000081fb58 in ServerLoop () at postmaster.c:1706
#5  0x000000000081f3f0 in PostmasterMain (argc=3, argv=0x256f320) at postmaster.c:1379
#6  0x00000000007469d4 in main (argc=3, argv=0x256f320) at main.c:228
(gdb) n
897             bool            save_log_statement_stats = log_statement_stats;
(gdb) 
898             bool            was_logged = false;
(gdb) 
905             debug_query_string = query_string;
(gdb) 
907             pgstat_report_activity(STATE_RUNNING, query_string);
(gdb) 
909             TRACE_POSTGRESQL_QUERY_START(query_string);
(gdb) 
915             if (save_log_statement_stats)
(gdb) 
925             start_xact_command();
(gdb) 
933             drop_unnamed_stmt();
(gdb) 
938             oldcontext = MemoryContextSwitchTo(MessageContext);
(gdb) 
944             parsetree_list = pg_parse_query(query_string);
(gdb) 
947             if (check_log_statement(parsetree_list))
(gdb) 
949                     ereport(LOG,
(gdb) 
953                     was_logged = true;
(gdb) 
959             MemoryContextSwitchTo(oldcontext);
(gdb) 
969             use_implicit_block = (list_length(parsetree_list) > 1);
(gdb) 
974             foreach(parsetree_item, parsetree_list)
(gdb) 
976                     RawStmt    *parsetree = lfirst_node(RawStmt, parsetree_item);
(gdb) 
977                     bool            snapshot_set = false;
(gdb) p *parsetree
$73 = {type = T_RawStmt, stmt = 0x25761a8, stmt_location = 0, stmt_len = 73}
(gdb) p *parsetree->stmt
$74 = {type = T_SelectStmt}
(gdb) p *((SelectStmt*)parsetree->stmt)
$75 = {type = T_SelectStmt, distinctClause = 0x0, intoClause = 0x0, targetList = 0x25754f8, fromClause = 0x2575628, whereClause = 0x2575ee0, groupClause = 0x0, havingClause = 0x0, windowClause = 0x0, 
  valuesLists = 0x0, sortClause = 0x0, limitOffset = 0x0, limitCount = 0x0, lockingClause = 0x0, withClause = 0x0, op = SETOP_NONE, all = false, larg = 0x0, rarg = 0x0}

/* 分析targetList結(jié)構(gòu) */
(gdb) p *((SelectStmt*)parsetree->stmt)->targetList
$76 = {type = T_List, length = 1, head = 0x25754d0, tail = 0x25754d0}
(gdb) p *((Node*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)
$77 = {type = T_ResTarget}
(gdb) p *((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)
$78 = {type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x25753c0, location = 7}
(gdb) p *((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val
$79 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)
$80 = {type = T_ColumnRef, fields = 0x2575440, location = 7}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields
$81 = {type = T_List, length = 1, head = 0x2575418, tail = 0x2575418}
(gdb) p *((Node*)((ColumnRef*)((ResTarget*)((SelectStmt*)parsetree->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)
$82 = {type = T_A_Star}

/* 分析fromClause結(jié)構(gòu) */
(gdb) p *((SelectStmt*)parsetree->stmt)->fromClause
$83 = {type = T_List, length = 2, head = 0x2575600, tail = 0x2575730} /* 兩個節(jié)點*/
(gdb) p *((Node*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)
$84 = {type = T_RangeVar}
(gdb) p *((RangeVar*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)
$85 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x2575530 "t_a", inh = true, relpersistence = 112 'p', alias = 0x25755c8, location = 15}
(gdb) p *((RangeVar*)((SelectStmt*)parsetree->stmt)->fromClause->head->data->ptr_value)->alias
$86 = {type = T_Alias, aliasname = 0x2575550 "a", colnames = 0x0}
(gdb) p *((Node*)((SelectStmt*)parsetree->stmt)->fromClause->head->next->data->ptr_value)
$87 = {type = T_RangeVar}
(gdb) p *((RangeVar*)((SelectStmt*)parsetree->stmt)->fromClause->head->next->data->ptr_value)
$88 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0, relname = 0x2575660 "t_b", inh = true, relpersistence = 112 'p', alias = 0x25756f8, location = 21}
(gdb) p *((RangeVar*)((SelectStmt*)parsetree->stmt)->fromClause->head->next->data->ptr_value)->alias
$89 = {type = T_Alias, aliasname = 0x2575680 "b", colnames = 0x0}

/* 分析whereClause結(jié)構(gòu) */
(gdb) p *((SelectStmt*)parsetree->stmt)->whereClause
$90 = {type = T_BoolExpr}
(gdb) p *((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)
$91 = {xpr = {type = T_BoolExpr}, boolop = AND_EXPR, args = 0x2575e80, location = 44}
(gdb) p *((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args
$92 = {type = T_List, length = 3, head = 0x2575eb8, tail = 0x2576180} /* 三個節(jié)點*/

/* 分析whereClause結(jié)構(gòu)瓣喊，第一個條件：a.id=b.id */
(gdb) p *((Node*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)
$93 = {type = T_A_Expr}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)
$95 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x2575aa0, lexpr = 0x2575820, rexpr = 0x2575970, location = 38}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->name
$96 = {type = T_List, length = 1, head = 0x2575a78, tail = 0x2575a78}
(gdb) p *((Node*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->name->head->data->ptr_value)
$97 = {type = T_String}
(gdb) p *((Value*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->name->head->data->ptr_value)
$98 = {type = T_String, val = {ival = 12092910, str = 0xb885ee "="}}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr
$99 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr)
$100 = {type = T_ColumnRef, fields = 0x25757e8, location = 34}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr)->fields
$101 = {type = T_List, length = 2, head = 0x2575880, tail = 0x25757c0}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$102 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$103 = {type = T_String, val = {ival = 39278424, str = 0x2575758 "a"}}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$104 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$105 = {type = T_String, val = {ival = 39278456, str = 0x2575778 "id"}}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->rexpr
$106 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->rexpr)
$107 = {type = T_ColumnRef, fields = 0x2575938, location = 39}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->rexpr)->fields
$108 = {type = T_List, length = 2, head = 0x25759d0, tail = 0x2575910}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->rexpr)->fields->head->data->ptr_value)
$109 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->rexpr)->fields->head->data->ptr_value)
$110 = {type = T_String, val = {ival = 39278760, str = 0x25758a8 "b"}}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->data->ptr_value)->rexpr)->fields->head->next->data->ptr_value)
$111 = {type = T_String, val = {ival = 39278792, str = 0x25758c8 "id"}}

/* 分析whereClause結(jié)構(gòu)坡慌，第二個條件：a.code=b.code */
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)
$112 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x2575e20, lexpr = 0x2575ba0, rexpr = 0x2575cf0, location = 54}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->name
$113 = {type = T_List, length = 1, head = 0x2575df8, tail = 0x2575df8}
(gdb) p *((Node*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->name->head->data->ptr_value)
$114 = {type = T_String}
(gdb) p *((Value*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->name->head->data->ptr_value)
$115 = {type = T_String, val = {ival = 12092910, str = 0xb885ee "="}}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->lexpr
$116 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->lexpr)
$117 = {type = T_ColumnRef, fields = 0x2575b68, location = 48}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->lexpr)->fields
$118 = {type = T_List, length = 2, head = 0x2575c00, tail = 0x2575b40}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$119 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$120 = {type = T_String, val = {ival = 39279320, str = 0x2575ad8 "a"}}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$121 = {type = T_String, val = {ival = 39279352, str = 0x2575af8 "code"}}

/* 分析whereClause結(jié)構(gòu)，第三個條件：a.id=8 */
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)
$122 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x2576148, lexpr = 0x2575fe0, rexpr = 0x2576068, location = 70}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->name
$123 = {type = T_List, length = 1, head = 0x2576120, tail = 0x2576120}
(gdb) p *((Node*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->name->head->data->ptr_value)
$124 = {type = T_String}
(gdb) p *((Value*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->name->head->data->ptr_value)
$125 = {type = T_String, val = {ival = 12092918, str = 0xb885f6 "<>"}}
(gdb) p *((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->lexpr
$126 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->lexpr)
$127 = {type = T_ColumnRef, fields = 0x2575fa8, location = 66}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->lexpr)->fields
$128 = {type = T_List, length = 2, head = 0x2576040, tail = 0x2575f80}
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$129 = {type = T_String}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->lexpr)->fields->head->data->ptr_value)
$130 = {type = T_String, val = {ival = 39280408, str = 0x2575f18 "a"}}
(gdb) p *((Value*)((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->lexpr)->fields->head->next->data->ptr_value)
$131 = {type = T_String, val = {ival = 39280440, str = 0x2575f38 "id"}}
(gdb) p *((ColumnRef*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->rexpr)
$132 = {type = T_A_Const, fields = 0xd8, location = 8}
(gdb) p *((A_Const*)((A_Expr*)((BoolExpr*)((SelectStmt*)parsetree->stmt)->whereClause)->args->head->next->next->data->ptr_value)->rexpr)
$133 = {type = T_A_Const, val = {type = T_Integer, val = {ival = 8, str = 0x8 <Address 0x8 out of bounds>}}, location = 72}
(gdb)

對應(yīng)的語法樹：


內(nèi)關(guān)聯(lián)語法樹

四藻三、兼容性改造

為了兼容oracle專有的外關(guān)聯(lián)語法洪橘，類似以下語法

select * 
from t_a a,t_b b 
where a.id=b.id(+) and a.code=b.code(+) 
    and a.id!=8;

通過改造語法分析器，上面的SQL對應(yīng)的語法樹為：


改造后的語法樹

然后在通過算法棵帽，將上面的語法樹修改為外關(guān)聯(lián)查詢語法樹


外關(guān)聯(lián)語法樹2

本語法樹就是標準的外關(guān)聯(lián)語法樹熄求，SQL查詢的后續(xù) 語義分析 及 生成執(zhí)行計劃均不需要修改，即可實現(xiàn)oracle的專有語法兼容逗概。

總結(jié)

通過分析原有的外關(guān)聯(lián)語法樹弟晚，及對應(yīng)的內(nèi)關(guān)聯(lián)語法樹，通過改造語法分析器和語法樹逾苫，達到兼容oracle的目的

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