Improve performance of EXPLAIN with large range tables.

As of 9.3, ruleutils.c goes to some lengths to ensure that table and column
aliases used in its output are unique.  Of course this takes more time than
was required before, which in itself isn't fatal.  However, EXPLAIN was set
up so that recalculation of the unique aliases was repeated for each
subexpression printed in a plan.  That results in O(N^2) time and memory
consumption for large plan trees, which did not happen in older branches.

Fortunately, the expensive work is the same across a whole plan tree,
so there is no need to repeat it; we can do most of the initialization
just once per query and re-use it for each subexpression.  This buys
back most (not all) of the performance loss since 9.2.

We need an extra ExplainState field to hold the precalculated deparse
context.  That's no problem in HEAD, but in the back branches, expanding
sizeof(ExplainState) seems risky because third-party extensions might
have local variables of that struct type.  So, in 9.4 and 9.3, introduce
an auxiliary struct to keep sizeof(ExplainState) the same.  We should
refactor the APIs to avoid such local variables in future, but that's
material for a separate HEAD-only commit.

Per gripe from Alexey Bashtanov.  Back-patch to 9.3 where the issue
was introduced.

Author: tglsfdc

src/backend/commands/explain.c

@@ -563,6 +563,8 @@ ExplainPrintPlan(ExplainState *es, QueryDesc *queryDesc)
 	es->rtable = queryDesc->plannedstmt->rtable;
 	ExplainPreScanNode(queryDesc->planstate, &rels_used);
 	es->rtable_names = select_rtable_names_for_explain(es->rtable, rels_used);
+	es->deparse_cxt = deparse_context_for_plan_rtable(es->rtable,
+													  es->rtable_names);
 	ExplainNode(queryDesc->planstate, NIL, NULL, NULL, es);
 }
 
@@ -1678,10 +1680,9 @@ show_plan_tlist(PlanState *planstate, List *ancestors, ExplainState *es)
 		return;
 
 	/* Set up deparsing context */
-	context = deparse_context_for_planstate((Node *) planstate,
-											ancestors,
-											es->rtable,
-											es->rtable_names);
+	context = set_deparse_context_planstate(es->deparse_cxt,
+											(Node *) planstate,
+											ancestors);
 	useprefix = list_length(es->rtable) > 1;
 
 	/* Deparse each result column (we now include resjunk ones) */
@@ -1710,10 +1711,9 @@ show_expression(Node *node, const char *qlabel,
 	char	   *exprstr;
 
 	/* Set up deparsing context */
-	context = deparse_context_for_planstate((Node *) planstate,
-											ancestors,
-											es->rtable,
-											es->rtable_names);
+	context = set_deparse_context_planstate(es->deparse_cxt,
+											(Node *) planstate,
+											ancestors);
 
 	/* Deparse the expression */
 	exprstr = deparse_expression(node, context, useprefix, false);
@@ -1855,10 +1855,9 @@ show_sort_group_keys(PlanState *planstate, const char *qlabel,
 		return;
 
 	/* Set up deparsing context */
-	context = deparse_context_for_planstate((Node *) planstate,
-											ancestors,
-											es->rtable,
-											es->rtable_names);
+	context = set_deparse_context_planstate(es->deparse_cxt,
+											(Node *) planstate,
+											ancestors);
 	useprefix = (list_length(es->rtable) > 1 || es->verbose);
 
 	for (keyno = 0; keyno < nkeys; keyno++)

src/backend/utils/adt/ruleutils.c

@@ -2520,7 +2520,43 @@ deparse_context_for(const char *aliasname, Oid relid)
 }
 
 /*
- * deparse_context_for_planstate	- Build deparse context for a plan
+ * deparse_context_for_plan_rtable - Build deparse context for a plan's rtable
+ *
+ * When deparsing an expression in a Plan tree, we use the plan's rangetable
+ * to resolve names of simple Vars.  The initialization of column names for
+ * this is rather expensive if the rangetable is large, and it'll be the same
+ * for every expression in the Plan tree; so we do it just once and re-use
+ * the result of this function for each expression.  (Note that the result
+ * is not usable until set_deparse_context_planstate() is applied to it.)
+ *
+ * In addition to the plan's rangetable list, pass the per-RTE alias names
+ * assigned by a previous call to select_rtable_names_for_explain.
+ */
+List *
+deparse_context_for_plan_rtable(List *rtable, List *rtable_names)
+{
+	deparse_namespace *dpns;
+
+	dpns = (deparse_namespace *) palloc0(sizeof(deparse_namespace));
+
+	/* Initialize fields that stay the same across the whole plan tree */
+	dpns->rtable = rtable;
+	dpns->rtable_names = rtable_names;
+	dpns->ctes = NIL;
+
+	/*
+	 * Set up column name aliases.  We will get rather bogus results for join
+	 * RTEs, but that doesn't matter because plan trees don't contain any join
+	 * alias Vars.
+	 */
+	set_simple_column_names(dpns);
+
+	/* Return a one-deep namespace stack */
+	return list_make1(dpns);
+}
+
+/*
+ * set_deparse_context_planstate	- Specify Plan node containing expression
  *
  * When deparsing an expression in a Plan tree, we might have to resolve
  * OUTER_VAR, INNER_VAR, or INDEX_VAR references.  To do this, the caller must
@@ -2533,43 +2569,34 @@ deparse_context_for(const char *aliasname, Oid relid)
  * fields, which won't contain INDEX_VAR Vars.)
  *
  * Note: planstate really ought to be declared as "PlanState *", but we use
- * "Node *" to avoid having to include execnodes.h in builtins.h.
+ * "Node *" to avoid having to include execnodes.h in ruleutils.h.
  *
  * The ancestors list is a list of the PlanState's parent PlanStates, the
  * most-closely-nested first.  This is needed to resolve PARAM_EXEC Params.
  * Note we assume that all the PlanStates share the same rtable.
  *
- * The plan's rangetable list must also be passed, along with the per-RTE
- * alias names assigned by a previous call to select_rtable_names_for_explain.
- * (We use the rangetable to resolve simple Vars, but the plan inputs are
- * necessary for Vars with special varnos.)
+ * Once this function has been called, deparse_expression() can be called on
+ * subsidiary expression(s) of the specified PlanState node.  To deparse
+ * expressions of a different Plan node in the same Plan tree, re-call this
+ * function to identify the new parent Plan node.
+ *
+ * The result is the same List passed in; this is a notational convenience.
  */
 List *
-deparse_context_for_planstate(Node *planstate, List *ancestors,
-							  List *rtable, List *rtable_names)
+set_deparse_context_planstate(List *dpcontext,
+							  Node *planstate, List *ancestors)
 {
 	deparse_namespace *dpns;
 
-	dpns = (deparse_namespace *) palloc0(sizeof(deparse_namespace));
-
-	/* Initialize fields that stay the same across the whole plan tree */
-	dpns->rtable = rtable;
-	dpns->rtable_names = rtable_names;
-	dpns->ctes = NIL;
-
-	/*
-	 * Set up column name aliases.  We will get rather bogus results for join
-	 * RTEs, but that doesn't matter because plan trees don't contain any join
-	 * alias Vars.
-	 */
-	set_simple_column_names(dpns);
+	/* Should always have one-entry namespace list for Plan deparsing */
+	Assert(list_length(dpcontext) == 1);
+	dpns = (deparse_namespace *) linitial(dpcontext);
 
 	/* Set our attention on the specific plan node passed in */
 	set_deparse_planstate(dpns, (PlanState *) planstate);
 	dpns->ancestors = ancestors;
 
-	/* Return a one-deep namespace stack */
-	return list_make1(dpns);
+	return dpcontext;
 }
 
 /*

src/include/commands/explain.h

@@ -41,6 +41,7 @@ typedef struct ExplainState
 	List	   *rtable_names;	/* alias names for RTEs */
 	int			indent;			/* current indentation level */
 	List	   *grouping_stack; /* format-specific grouping state */
+	List	   *deparse_cxt;	/* context list for deparsing expressions */
 } ExplainState;
 
 /* Hook for plugins to get control in ExplainOneQuery() */

src/include/utils/ruleutils.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * src/include/ruleutils.h
+ * src/include/utils/ruleutils.h
  *
  *-------------------------------------------------------------------------
  */
@@ -25,8 +25,9 @@ extern char *pg_get_constraintdef_string(Oid constraintId);
 extern char *deparse_expression(Node *expr, List *dpcontext,
 				   bool forceprefix, bool showimplicit);
 extern List *deparse_context_for(const char *aliasname, Oid relid);
-extern List *deparse_context_for_planstate(Node *planstate, List *ancestors,
-							  List *rtable, List *rtable_names);
+extern List *deparse_context_for_plan_rtable(List *rtable, List *rtable_names);
+extern List *set_deparse_context_planstate(List *dpcontext,
+							  Node *planstate, List *ancestors);
 extern List *select_rtable_names_for_explain(List *rtable,
 								Bitmapset *rels_used);
 extern char *generate_collation_name(Oid collid);