这里我们分析一下查询重写的过程，主要分析视图的查询重写的过程。通过以下语句进行分析：
sql
create table t1(a int, b int);
insert into t1 values(1,1);
-- 创建视图
create view vt1 as select * from t1;
-- 查询
select * from vt1;查询重写过程分析
select * from vt1生成一个抽象语法树SelectStmt。其中视图名vt1通过RangeVar节点表示。再由SelectStmt经由语义分析转为查询树Query。
语法解析部分的源码主流程如下：
c
exec_simple_query
--> pg_parse_query
--> raw_parser
--> base_yyparse
--> pg_analyze_and_rewrite
--> parse_analyze
--> transformStmt
--> transformFromClause
--> transformFromClauseItem
--> pg_rewrite_query
--> QueryRewrite // 查询重写，视图重写为子查询是在这里发生的。
// Step 1: Apply all non-SELECT rules possibly getting 0 or many queries
--> RewriteQuery
// Step 2: Apply all the RIR rules on each query
--> fireRIRrules
--> ApplyRetrieveRule // expand an ON SELECT rule
--> fireRIRrules // Recursively expand any view references inside the view.
/* Step 3: Determine which, if any, of the resulting queries is supposed to set
the command-result tag; and update the canSetTag fields accordingly. */
--> pg_plan_queries视图定义的规则展开的关键函数为ApplyRetrieveRule，我们重点分析一下：
c
/* ApplyRetrieveRule - expand an ON SELECT rule*/
static Query *ApplyRetrieveRule(Query *parsetree, RewriteRule *rule, int rt_index, Relation relation, List *activeRIRs)
{
Query *rule_action;
RangeTblEntry *rte, *subrte;
RowMarkClause *rc;
// ...
/*
* Make a modifiable copy of the view query, and acquire needed locks on
* the relations it mentions. Force at least RowShareLock for all such
* rels if there's a FOR [KEY] UPDATE/SHARE clause affecting this view.
*/
rule_action = copyObject(linitial(rule->actions));
AcquireRewriteLocks(rule_action, true, (rc != NULL));
/*
* Recursively expand any view references inside the view.
*
* Note: this must happen after markQueryForLocking. That way, any UPDATE
* permission bits needed for sub-views are initially applied to their
* RTE_RELATION RTEs by markQueryForLocking, and then transferred to their
* OLD rangetable entries by the action below (in a recursive call of this
* routine).
*/
rule_action = fireRIRrules(rule_action, activeRIRs); // 规则应用
/*
* Now, plug the view query in as a subselect, converting the relation's
* original RTE to a subquery RTE.
*/
rte = rt_fetch(rt_index, parsetree->rtable);
rte->rtekind = RTE_SUBQUERY; // 视图会通过子查询进行替换，对应的就是由RTE_SUBQUERY替换掉原先的RTE_RELATION
rte->subquery = rule_action;
rte->security_barrier = RelationIsSecurityView(relation);
/* Clear fields that should not be set in a subquery RTE */
rte->relid = InvalidOid;
rte->relkind = 0;
rte->rellockmode = 0;
rte->tablesample = NULL;
rte->inh = false; /* must not be set for a subquery */
/*
* We move the view's permission check data down to its rangetable. The
* checks will actually be done against the OLD entry therein.
*/
subrte = rt_fetch(PRS2_OLD_VARNO, rule_action->rtable);
Assert(subrte->relid == relation->rd_id);
subrte->requiredPerms = rte->requiredPerms;
subrte->checkAsUser = rte->checkAsUser;
subrte->selectedCols = rte->selectedCols;
subrte->insertedCols = rte->insertedCols;
subrte->updatedCols = rte->updatedCols;
subrte->extraUpdatedCols = rte->extraUpdatedCols;
// ...
return parsetree;
}可以看到，最核心的部分就是将RangeTblEntry由视图转换为子查询。
分析完源码，我们再回过头来研究一下其过程：
执行创建视图命令后create view vt1 as select * from t1;，向pg_rewrite系统表中插入了一套数据：
sql
postgres@postgres=# select * from pg_rewrite order by oid desc limit 1;
oid | 16390
rulename | _RETURN
ev_class | 16387 -- 视图vt1的表oid
ev_type | 1
ev_enabled | O
is_instead | t -- instead规则
ev_qual | <>
ev_action | ({QUERY :commandType 1 :querySource 0 :canSetTag true :utilityStmt <> :resultRelation 0 :hasAggs false :hasWindowFuncs false :hasTargetSRFs false :hasSubLinks false :hasDistinctOn false :hasRecursive false :hasModifyingCTE false :hasForUpdate false :hasRowSecurity false :cteList <> :rtable ({RTE :alias {ALIAS :aliasname old :colnames <>} :eref {ALIAS :aliasname old :colnames ("a" "b")} :rtekind 0 :relid 16387 :relkind v :rellockmode 1 :tablesample <> :lateral false :inh false :inFromCl false :requiredPerms 0 :checkAsUser 0 :selectedCols (b) :insertedCols (b) :updatedCols (b) :extraUpdatedCols (b) :securityQuals <>} {RTE :alias {ALIAS :aliasname new :colnames <>} :eref {ALIAS :aliasname new :colnames ("a" "b")} :rtekind 0 :relid 16387 :relkind v :rellockmode 1 :tablesample <> :lateral false :inh false :inFromCl false :requiredPerms 0 :checkAsUser 0 :selectedCols (b) :insertedCols (b) :updatedCols (b) :extraUpdatedCols (b) :securityQuals <>} {RTE :alias <> :eref {ALIAS :aliasname t1 :colnames ("a" "b")} :rtekind 0 :relid 16384 :relkind r :rellockmode 1 :tablesample <> :lateral false :inh true :inFromCl true :requiredPerms 2 :checkAsUser 0 :selectedCols (b 8 9) :insertedCols (b) :updatedCols (b) :extraUpdatedCols (b) :securityQuals <>}) :jointree {FROMEXPR :fromlist ({RANGETBLREF :rtindex 3}) :quals <>} :targetList ({TARGETENTRY :expr {VAR :varno 3 :varattno 1 :vartype 23 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnosyn 3 :varattnosyn 1 :location 26} :resno 1 :resname a :ressortgroupref 0 :resorigtbl 16384 :resorigcol 1 :resjunk false} {TARGETENTRY :expr {VAR :varno 3 :varattno 2 :vartype 23 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnosyn 3 :varattnosyn 2 :location 26} :resno 2 :resname b :ressortgroupref 0 :resorigtbl 16384 :resorigcol 2 :resjunk false}) :override 0 :onConflict <> :returningList <> :groupClause <> :groupingSets <> :havingQual <> :windowClause <> :distinctClause <> :sortClause <> :limitOffset <> :limitCount <> :limitOption 0 :rowMarks <> :setOperations <> :constraintDeps <> :withCheckOptions <>})建表以及创建视图后，会向pg_class系统表插入如下的信息。
sql
postgres@postgres=# select oid,relname,relkind,relhasrules,relrewrite from pg_class where relname='vt1';
-[ RECORD 1 ]------
oid | 16387 -- 表OID
relname | vt1 -- 视图名
relkind | v -- 表示视图
relhasrules | t -- 表是否定义了规则
relrewrite | 0
postgres@postgres=# select oid,relname,relkind,relhasrules,relrewrite from pg_class where relname='t1';
-[ RECORD 1 ]------
oid | 16384 -- 表OID
relname | t1 -- 表名
relkind | r -- 表示是普通表
relhasrules | f -- 表是否定义了规则
relrewrite | 0当执行select * from vt1时，先查pg_class系统表，找到表vt1类型为视图，同时该表定义了规则。查看pg_rewrite系统表，找到该表定义的规则的类型以及行为，应用规则。