Share transition state between different aggregates when possible.

If there are two different aggregates in the query with same inputs, and
the aggregates have the same initial condition and transition function,
only calculate the state value once, and only call the final functions
separately. For example, AVG(x) and SUM(x) aggregates have the same
transition function, which accumulates the sum and number of input tuples.
For a query like "SELECT AVG(x), SUM(x) FROM x", we can therefore
accumulate the state function only once, which gives a nice speedup.

David Rowley, reviewed and edited by me.

1 files changed, 1 insertions, 21 deletions

diff --git a/src/backend/executor/execQual.c b/src/backend/executor/execQual.c
index 16bc8fa5f6c..29f058ce5cb 100644
--- a/src/backend/executor/execQual.c
+++ b/src/backend/executor/execQual.c
@@ -4487,35 +4487,15 @@ ExecInitExpr(Expr *node, PlanState *parent)
 			break;
 		case T_Aggref:
 			{
-				Aggref	   *aggref = (Aggref *) node;
 				AggrefExprState *astate = makeNode(AggrefExprState);
 
 				astate->xprstate.evalfunc = (ExprStateEvalFunc) ExecEvalAggref;
 				if (parent && IsA(parent, AggState))
 				{
 					AggState   *aggstate = (AggState *) parent;
-					int			naggs;
 
 					aggstate->aggs = lcons(astate, aggstate->aggs);
-					naggs = ++aggstate->numaggs;
-
-					astate->aggdirectargs = (List *) ExecInitExpr((Expr *) aggref->aggdirectargs,
-																  parent);
-					astate->args = (List *) ExecInitExpr((Expr *) aggref->args,
-														 parent);
-					astate->aggfilter = ExecInitExpr(aggref->aggfilter,
-													 parent);
-
-					/*
-					 * Complain if the aggregate's arguments contain any
-					 * aggregates; nested agg functions are semantically
-					 * nonsensical.  (This should have been caught earlier,
-					 * but we defend against it here anyway.)
-					 */
-					if (naggs != aggstate->numaggs)
-						ereport(ERROR,
-								(errcode(ERRCODE_GROUPING_ERROR),
-						errmsg("aggregate function calls cannot be nested")));
+					aggstate->numaggs++;
 				}
 				else
 				{