本节简单介绍了PG查询优化表达式预处理中的生成子链接执行计划、使用Param替换上层Vars以及转换表达式为隐式AND格式（implicit-AND format）。
一、主函数
主函数preprocess_expression先前章节也介绍过，在此函数中调用了生成子链接执行计划、使用Param替换上层Vars以及转换表达式为隐式AND格式（implicit-AND format）等相关子函数。
preprocess_expression
/*
* preprocess_expression
* Do subquery_planner's preprocessing work for an expression,
* which can be a targetlist, a WHERE clause (including JOIN/ON
* conditions), a HAVING clause, or a few other things.
*/
static Node *
preprocess_expression(PlannerInfo *root, Node *expr, int kind)
{
//...
/* Expand SubLinks to SubPlans */
if (root->parse->hasSubLinks)//扩展子链接为子计划
expr = SS_process_sublinks(root, expr, (kind == EXPRKIND_QUAL));
/*
* XXX do not insert anything here unless you have grokked the comments in
* SS_replace_correlation_vars ...
*/
/* Replace uplevel vars with Param nodes (this IS possible in VALUES) */
if (root->query_level > 1)
expr = SS_replace_correlation_vars(root, expr);//使用Param节点替换上层的Vars
/*
* If it's a qual or havingQual, convert it to implicit-AND format. (We
* don't want to do this before eval_const_expressions, since the latter
* would be unable to simplify a top-level AND correctly. Also,
* SS_process_sublinks expects explicit-AND format.)
*/
if (kind == EXPRKIND_QUAL)//转换为隐式AND格式
expr = (Node *) make_ands_implicit((Expr *) expr);
return expr;
}
二、生成子链接执行计划
先前的章节已介绍了上拉子链接的相关处理过程,对于不能上拉的子链接,PG会生成子执行计划.对于会生成常量的子链接,则会把生成的常量记录在Param中,在需要的时候由父查询使用.
例1:以下的子链接,PG会生成子计划,并把子链接的结果物化(Materialize)提升整体性能.
testdb=# explain verbose select * from t_dwxx where dwbh > all (select b.dwbh from t_grxx b);
QUERY PLAN
---------------------------------------------------------------------------------
Seq Scan on public.t_dwxx (cost=0.00..1498.00 rows=80 width=474)
Output: t_dwxx.dwmc, t_dwxx.dwbh, t_dwxx.dwdz
Filter: (SubPlan 1)
SubPlan 1
-> Materialize (cost=0.00..17.35 rows=490 width=38)
Output: b.dwbh
-> Seq Scan on public.t_grxx b (cost=0.00..14.90 rows=490 width=38)
Output: b.dwbh
(8 rows)
例2:以下的子链接,PG会把生成的常量记录在Param中(注意生成的参数:$0)
testdb=# explain verbose select * from t_dwxx a where exists (select max(b.dwbh) from t_grxx b);
QUERY PLAN
---------------------------------------------------------------------------------
Result (cost=16.14..27.73 rows=160 width=474)
Output: a.dwmc, a.dwbh, a.dwdz
One-Time Filter: $0
InitPlan 1 (returns $0)
-> Aggregate (cost=16.12..16.14 rows=1 width=32)
Output: max((b.dwbh)::text)
-> Seq Scan on public.t_grxx b (cost=0.00..14.90 rows=490 width=38)
Output: b.dwbh, b.grbh, b.xm, b.nl
-> Seq Scan on public.t_dwxx a (cost=16.14..27.73 rows=160 width=474)
Output: a.dwmc, a.dwbh, a.dwdz
(10 rows)
源代码如下:
SS_process_sublinks
/*
* Expand SubLinks to SubPlans in the given expression.
*
* The isQual argument tells whether or not this expression is a WHERE/HAVING
* qualifier expression. If it is, any sublinks appearing at top level need
* not distinguish FALSE from UNKNOWN return values.
*/
Node *
SS_process_sublinks(PlannerInfo *root, Node *expr, bool isQual)
{
process_sublinks_context context;
context.root = root;
context.isTopQual = isQual;
return process_sublinks_mutator(expr, &context);//调用XX_mutator函数遍历并处理
}
static Node *
process_sublinks_mutator(Node *node, process_sublinks_context *context)
{
process_sublinks_context locContext;
locContext.root = context->root;
if (node == NULL)
return NULL;
if (IsA(node, SubLink))//子链接
{
SubLink *sublink = (SubLink *) node;
Node *testexpr;
/*
* First, recursively process the lefthand-side expressions, if any.
* They're not top-level anymore.
*/
locContext.isTopQual = false;
testexpr = process_sublinks_mutator(sublink->testexpr, &locContext);
/*
* Now build the SubPlan node and make the expr to return.
*/
return make_subplan(context->root,
(Query *) sublink->subselect,
sublink->subLinkType,
sublink->subLinkId,
testexpr,
context->isTopQual);//生成子执行计划,与整体的执行计划类似
}
/*
* Don't recurse into the arguments of an outer PHV or aggregate here. Any
* SubLinks in the arguments have to be dealt with at the outer query
* level; they'll be handled when build_subplan collects the PHV or Aggref
* into the arguments to be passed down to the current subplan.
*/
if (IsA(node, PlaceHolderVar))
{
if (((PlaceHolderVar *) node)->phlevelsup > 0)
return node;
}
else if (IsA(node, Aggref))
{
if (((Aggref *) node)->agglevelsup > 0)
return node;
}
/*
* We should never see a SubPlan expression in the input (since this is
* the very routine that creates 'em to begin with). We shouldn't find
* ourselves invoked directly on a Query, either.
*/
Assert(!IsA(node, SubPlan));
Assert(!IsA(node, AlternativeSubPlan));
Assert(!IsA(node, Query));
/*
* Because make_subplan() could return an AND or OR clause, we have to
* take steps to preserve AND/OR flatness of a qual. We assume the input
* has been AND/OR flattened and so we need no recursion here.
*
* (Due to the coding here, we will not get called on the List subnodes of
* an AND; and the input is *not* yet in implicit-AND format. So no check
* is needed for a bare List.)
*
* Anywhere within the top-level AND/OR clause structure, we can tell
* make_subplan() that NULL and FALSE are interchangeable. So isTopQual
* propagates down in both cases. (Note that this is unlike the meaning
* of "top level qual" used in most other places in Postgres.)
*/
if (and_clause(node))//AND语句
{
List *newargs = NIL;
ListCell *l;
/* Still at qual top-level */
locContext.isTopQual = context->isTopQual;
foreach(l, ((BoolExpr *) node)->args)
{
Node *newarg;
newarg = process_sublinks_mutator(lfirst(l), &locContext);
if (and_clause(newarg))
newargs = list_concat(newargs, ((BoolExpr *) newarg)->args);
else
newargs = lappend(newargs, newarg);
}
return (Node *) make_andclause(newargs);
}
if (or_clause(node))//OR语句
{
List *newargs = NIL;
ListCell *l;
/* Still at qual top-level */
locContext.isTopQual = context->isTopQual;
foreach(l, ((BoolExpr *) node)->args)
{
Node *newarg;
newarg = process_sublinks_mutator(lfirst(l), &locContext);
if (or_clause(newarg))
newargs = list_concat(newargs, ((BoolExpr *) newarg)->args);
else
newargs = lappend(newargs, newarg);
}
return (Node *) make_orclause(newargs);
}
/*
* If we recurse down through anything other than an AND or OR node, we
* are definitely not at top qual level anymore.
*/
locContext.isTopQual = false;
return expression_tree_mutator(node,
process_sublinks_mutator,
(void *) &locContext);
}
三、使用Param替换上层变量
SQL例子参考"二、生成子链接执行计划"中的例2,这也是使用Param替代Var的一个例子.
源代码如下:
/*
* Replace correlation vars (uplevel vars) with Params.
*
* Uplevel PlaceHolderVars and aggregates are replaced, too.
*
* Note: it is critical that this runs immediately after SS_process_sublinks.
* Since we do not recurse into the arguments of uplevel PHVs and aggregates,
* they will get copied to the appropriate subplan args list in the parent
* query with uplevel vars not replaced by Params, but only adjusted in level
* (see replace_outer_placeholdervar and replace_outer_agg). That's exactly
* what we want for the vars of the parent level --- but if a PHV's or
* aggregate's argument contains any further-up variables, they have to be
* replaced with Params in their turn. That will happen when the parent level
* runs SS_replace_correlation_vars. Therefore it must do so after expanding
* its sublinks to subplans. And we don't want any steps in between, else
* those steps would never get applied to the argument expressions, either in
* the parent or the child level.
*
* Another fairly tricky thing going on here is the handling of SubLinks in
* the arguments of uplevel PHVs/aggregates. Those are not touched inside the
* intermediate query level, either. Instead, SS_process_sublinks recurses on
* them after copying the PHV or Aggref expression into the parent plan level
* (this is actually taken care of in build_subplan).
*/
Node *
SS_replace_correlation_vars(PlannerInfo *root, Node *expr)
{
/* No setup needed for tree walk, so away we go */
//调用XX_mutator遍历处理
return replace_correlation_vars_mutator(expr, root);
}
static Node *
replace_correlation_vars_mutator(Node *node, PlannerInfo *root)
{
if (node == NULL)
return NULL;
if (IsA(node, Var))//Var
{
if (((Var *) node)->varlevelsup > 0)
return (Node *) replace_outer_var(root, (Var *) node);//使用Param替换
}
if (IsA(node, PlaceHolderVar))
{
if (((PlaceHolderVar *) node)->phlevelsup > 0)
return (Node *) replace_outer_placeholdervar(root,
(PlaceHolderVar *) node);
}
if (IsA(node, Aggref))
{
if (((Aggref *) node)->agglevelsup > 0)
return (Node *) replace_outer_agg(root, (Aggref *) node);
}
if (IsA(node, GroupingFunc))
{
if (((GroupingFunc *) node)->agglevelsup > 0)
return (Node *) replace_outer_grouping(root, (GroupingFunc *) node);
}
return expression_tree_mutator(node,
replace_correlation_vars_mutator,
(void *) root);
}
/*
* Generate a Param node to replace the given Var,
* which is expected to have varlevelsup > 0 (ie, it is not local).
*/
static Param *
replace_outer_var(PlannerInfo *root, Var *var)//构造Param替换Var
{
Param *retval;
int i;
Assert(var->varlevelsup > 0 && var->varlevelsup < root->query_level);
/* Find the Var in the appropriate plan_params, or add it if not present */
i = assign_param_for_var(root, var);
retval = makeNode(Param);
retval->paramkind = PARAM_EXEC;
retval->paramid = i;
retval->paramtype = var->vartype;
retval->paramtypmod = var->vartypmod;
retval->paramcollid = var->varcollid;
retval->location = var->location;
return retval;
}
四、转换表达式为隐式AND格式
源码如下:
List *
make_ands_implicit(Expr *clause)
{
/*
* NB: because the parser sets the qual field to NULL in a query that has
* no WHERE clause, we must consider a NULL input clause as TRUE, even
* though one might more reasonably think it FALSE. Grumble. If this
* causes trouble, consider changing the parser's behavior.
*/
if (clause == NULL)//如为NULL,返回空指针
return NIL; /* NULL -> NIL list == TRUE */
else if (and_clause((Node *) clause))//AND语句,直接返回AND中的args参数
return ((BoolExpr *) clause)->args;
else if (IsA(clause, Const) &&
!((Const *) clause)->constisnull &&
DatumGetBool(((Const *) clause)->constvalue))
return NIL; /* 常量TRUE ,返回空指针constant TRUE input -> NIL list */
else
return list_make1(clause);//返回List
}
五、参考资料
subselect.c
clauses.c