Expression evaluation based aggregate transition invocation.

Previously aggregate transition and combination functions were invoked
by special case code in nodeAgg.c, evaluating input and filters
separately using the expression evaluation machinery. That turns out
to not be great for performance for several reasons:

- repeated expression evaluations have some cost
- the transition functions invocations are poorly predicted, as
  commonly there are multiple aggregates in a query, resulting in the
  same call-stack invoking different functions.
- filter and input computation had to be done separately
- the special case code made it hard to implement JITing of the whole
  transition function invocation

Address this by building one large expression that computes input,
evaluates filters, and invokes transition functions.

This leads to moderate speedups in queries bottlenecked by aggregate
computations, and enables large speedups for similar cases once JITing
is done.

There's potential for further improvement:
- It'd be nice if we could simplify the somewhat expensive
  aggstate->all_pergroups lookups.
- right now there's still an advance_transition_function invocation in
  nodeAgg.c, leading to some code duplication.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

1 files changed, 4 insertions, 1 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 2a4f7407a16..4bb5cb163d7 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1850,10 +1850,13 @@ typedef struct AggState
 	/* these fields are used in AGG_HASHED and AGG_MIXED modes: */
 	bool		table_filled;	/* hash table filled yet? */
 	int			num_hashes;
-	AggStatePerHash perhash;
+	AggStatePerHash perhash;	/* array of per-hashtable data */
 	AggStatePerGroup *hash_pergroup;	/* grouping set indexed array of
 										 * per-group pointers */
+
 	/* support for evaluation of agg input expressions: */
+	AggStatePerGroup *all_pergroups;	/* array of first ->pergroups, than
+										 * ->hash_pergroup */
 	ProjectionInfo *combinedproj;	/* projection machinery */
 } AggState;