Faster expression evaluation and targetlist projection.

This replaces the old, recursive tree-walk based evaluation, with
non-recursive, opcode dispatch based, expression evaluation.
Projection is now implemented as part of expression evaluation.

This both leads to significant performance improvements, and makes
future just-in-time compilation of expressions easier.

The speed gains primarily come from:
- non-recursive implementation reduces stack usage / overhead
- simple sub-expressions are implemented with a single jump, without
  function calls
- sharing some state between different sub-expressions
- reduced amount of indirect/hard to predict memory accesses by laying
  out operation metadata sequentially; including the avoidance of
  nearly all of the previously used linked lists
- more code has been moved to expression initialization, avoiding
  constant re-checks at evaluation time

Future just-in-time compilation (JIT) has become easier, as
demonstrated by released patches intended to be merged in a later
release, for primarily two reasons: Firstly, due to a stricter split
between expression initialization and evaluation, less code has to be
handled by the JIT. Secondly, due to the non-recursive nature of the
generated "instructions", less performance-critical code-paths can
easily be shared between interpreted and compiled evaluation.

The new framework allows for significant future optimizations. E.g.:
- basic infrastructure for to later reduce the per executor-startup
  overhead of expression evaluation, by caching state in prepared
  statements.  That'd be helpful in OLTPish scenarios where
  initialization overhead is measurable.
- optimizing the generated "code". A number of proposals for potential
  work has already been made.
- optimizing the interpreter. Similarly a number of proposals have
  been made here too.

The move of logic into the expression initialization step leads to some
backward-incompatible changes:
- Function permission checks are now done during expression
  initialization, whereas previously they were done during
  execution. In edge cases this can lead to errors being raised that
  previously wouldn't have been, e.g. a NULL array being coerced to a
  different array type previously didn't perform checks.
- The set of domain constraints to be checked, is now evaluated once
  during expression initialization, previously it was re-built
  every time a domain check was evaluated. For normal queries this
  doesn't change much, but e.g. for plpgsql functions, which caches
  ExprStates, the old set could stick around longer.  The behavior
  around might still change.

Author: Andres Freund, with significant changes by Tom Lane,
	changes by Heikki Linnakangas
Reviewed-By: Tom Lane, Heikki Linnakangas
Discussion: https://postgr.es/m/20161206034955.bh33paeralxbtluv@alap3.anarazel.de

1 files changed, 16 insertions, 18 deletions

diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index 3207ee460c2..471acc4b3ec 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -1639,7 +1639,7 @@ project_aggregates(AggState *aggstate)
 	/*
 	 * Check the qual (HAVING clause); if the group does not match, ignore it.
 	 */
-	if (ExecQual(aggstate->ss.ps.qual, econtext, false))
+	if (ExecQual(aggstate->ss.ps.qual, econtext))
 	{
 		/*
 		 * Form and return projection tuple using the aggregate results and
@@ -2501,18 +2501,17 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 	/*
 	 * initialize child expressions
 	 *
-	 * Note: ExecInitExpr finds Aggrefs for us, and also checks that no aggs
-	 * contain other agg calls in their arguments.  This would make no sense
-	 * under SQL semantics anyway (and it's forbidden by the spec). Because
-	 * that is true, we don't need to worry about evaluating the aggs in any
-	 * particular order.
+	 * We rely on the parser to have checked that no aggs contain other agg
+	 * calls in their arguments.  This would make no sense under SQL semantics
+	 * (and it's forbidden by the spec).  Because it is true, we don't need to
+	 * worry about evaluating the aggs in any particular order.
+	 *
+	 * Note: execExpr.c finds Aggrefs for us, and adds their AggrefExprState
+	 * nodes to aggstate->aggs.  Aggrefs in the qual are found here; Aggrefs
+	 * in the targetlist are found during ExecAssignProjectionInfo, below.
 	 */
-	aggstate->ss.ps.targetlist = (List *)
-		ExecInitExpr((Expr *) node->plan.targetlist,
-					 (PlanState *) aggstate);
-	aggstate->ss.ps.qual = (List *)
-		ExecInitExpr((Expr *) node->plan.qual,
-					 (PlanState *) aggstate);
+	aggstate->ss.ps.qual =
+		ExecInitQual(node->plan.qual, (PlanState *) aggstate);
 
 	/*
 	 * Initialize child nodes.
@@ -2540,7 +2539,7 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 	ExecAssignProjectionInfo(&aggstate->ss.ps, NULL);
 
 	/*
-	 * get the count of aggregates in targetlist and quals
+	 * We should now have found all Aggrefs in the targetlist and quals.
 	 */
 	numaggs = aggstate->numaggs;
 	Assert(numaggs == list_length(aggstate->aggs));
@@ -2724,7 +2723,7 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 	foreach(l, aggstate->aggs)
 	{
 		AggrefExprState *aggrefstate = (AggrefExprState *) lfirst(l);
-		Aggref	   *aggref = (Aggref *) aggrefstate->xprstate.expr;
+		Aggref	   *aggref = aggrefstate->aggref;
 		AggStatePerAgg peragg;
 		AggStatePerTrans pertrans;
 		int			existing_aggno;
@@ -3024,11 +3023,10 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 	/* and then create a projection for that targetlist */
 	aggstate->evaldesc = ExecTypeFromTL(combined_inputeval, false);
 	aggstate->evalslot = ExecInitExtraTupleSlot(estate);
-	combined_inputeval = (List *) ExecInitExpr((Expr *) combined_inputeval,
-											   (PlanState *) aggstate);
 	aggstate->evalproj = ExecBuildProjectionInfo(combined_inputeval,
 												 aggstate->tmpcontext,
 												 aggstate->evalslot,
+												 &aggstate->ss.ps,
 												 NULL);
 	ExecSetSlotDescriptor(aggstate->evalslot, aggstate->evaldesc);
 
@@ -3206,8 +3204,8 @@ build_pertrans_for_aggref(AggStatePerTrans pertrans,
 	naggs = aggstate->numaggs;
 	pertrans->aggfilter = ExecInitExpr(aggref->aggfilter,
 									   (PlanState *) aggstate);
-	pertrans->aggdirectargs = (List *) ExecInitExpr((Expr *) aggref->aggdirectargs,
-													(PlanState *) aggstate);
+	pertrans->aggdirectargs = ExecInitExprList(aggref->aggdirectargs,
+											   (PlanState *) aggstate);
 
 	/*
 	 * Complain if the aggregate's arguments contain any aggregates; nested