expression eval: Reduce number of steps for agg transition invocations.

Do so by combining the various steps that are part of aggregate
transition function invocation into one larger step. As some of the
current steps are only necessary for some aggregates, have one variant
of the aggregate transition step for each possible combination.

To avoid further manual copies of code in the different transition
step implementations, move most of the code into helper functions
marked as "always inline".

The benefit of this change is an increase in performance when
aggregating lots of rows. This comes in part due to the reduced number
of indirect jumps due to the reduced number of steps, and in part by
reducing redundant setup code across steps. This mainly benefits
interpreted execution, but the code generated by JIT is also improved
a bit.

As a nice side-effect it also ends up making the code a bit simpler.

A small additional optimization is removing the need to set
aggstate->curaggcontext before calling ExecAggInitGroup, choosing to
instead passign curaggcontext as an argument. It was, in contrast to
other aggregate related functions, only needed to fetch a memory
context to copy the transition value into.

Author: Andres Freund
Discussion:
   https://postgr.es/m/20191023163849.sosqbfs5yenocez3@alap3.anarazel.de
   https://postgr.es/m/5c371df7cee903e8cd4c685f90c6c72086d3a2dc.camel@j-davis.com

Author: anarazel

src/backend/executor/execExpr.c

@@ -3229,8 +3229,6 @@ ExecBuildAggTransCall(ExprState *state, AggState *aggstate,
 					  FunctionCallInfo fcinfo, AggStatePerTrans pertrans,
 					  int transno, int setno, int setoff, bool ishash)
 {
-	int			adjust_init_jumpnull = -1;
-	int			adjust_strict_jumpnull = -1;
 	ExprContext *aggcontext;
 
 	if (ishash)
@@ -3239,52 +3237,61 @@ ExecBuildAggTransCall(ExprState *state, AggState *aggstate,
 		aggcontext = aggstate->aggcontexts[setno];
 
 	/*
+	 * Determine appropriate transition implementation.
+	 *
+	 * For non-ordered aggregates:
+	 *
 	 * If the initial value for the transition state doesn't exist in the
 	 * pg_aggregate table then we will let the first non-NULL value returned
 	 * from the outer procNode become the initial value. (This is useful for
 	 * aggregates like max() and min().) The noTransValue flag signals that we
-	 * still need to do this.
+	 * need to do so. If true, generate a
+	 * EEOP_AGG_INIT_STRICT_PLAIN_TRANS{,_BYVAL} step. This step also needs to
+	 * do the work described next:
+	 *
+	 * If the function is strict, but does have an initial value, choose
+	 * EEOP_AGG_STRICT_PLAIN_TRANS{,_BYVAL}, which skips the transition
+	 * function if the transition value has become NULL (because a previous
+	 * transition function returned NULL). This step also needs to do the work
+	 * described next:
+	 *
+	 * Otherwise we call EEOP_AGG_PLAIN_TRANS{,_BYVAL}, which does not have to
+	 * perform either of the above checks.
+	 *
+	 * Having steps with overlapping responsibilities is not nice, but
+	 * aggregations are very performance sensitive, making this worthwhile.
+	 *
+	 * For ordered aggregates:
+	 *
+	 * Only need to choose between the faster path for a single orderred
+	 * column, and the one between multiple columns. Checking strictness etc
+	 * is done when finalizing the aggregate. See
+	 * process_ordered_aggregate_{single, multi} and
+	 * advance_transition_function.
 	 */
-	if (pertrans->numSortCols == 0 &&
-		fcinfo->flinfo->fn_strict &&
-		pertrans->initValueIsNull)
-	{
-		scratch->opcode = EEOP_AGG_INIT_TRANS;
-		scratch->d.agg_init_trans.pertrans = pertrans;
-		scratch->d.agg_init_trans.setno = setno;
-		scratch->d.agg_init_trans.setoff = setoff;
-		scratch->d.agg_init_trans.transno = transno;
-		scratch->d.agg_init_trans.aggcontext = aggcontext;
-		scratch->d.agg_init_trans.jumpnull = -1;	/* adjust later */
-		ExprEvalPushStep(state, scratch);
-
-		/* see comment about jumping out below */
-		adjust_init_jumpnull = state->steps_len - 1;
-	}
-
-	if (pertrans->numSortCols == 0 &&
-		fcinfo->flinfo->fn_strict)
+	if (pertrans->numSortCols == 0)
 	{
-		scratch->opcode = EEOP_AGG_STRICT_TRANS_CHECK;
-		scratch->d.agg_strict_trans_check.setno = setno;
-		scratch->d.agg_strict_trans_check.setoff = setoff;
-		scratch->d.agg_strict_trans_check.transno = transno;
-		scratch->d.agg_strict_trans_check.jumpnull = -1;	/* adjust later */
-		ExprEvalPushStep(state, scratch);
-
-		/*
-		 * Note, we don't push into adjust_bailout here - those jump to the
-		 * end of all transition value computations. Here a single transition
-		 * value is NULL, so just skip processing the individual value.
-		 */
-		adjust_strict_jumpnull = state->steps_len - 1;
+		if (pertrans->transtypeByVal)
+		{
+			if (fcinfo->flinfo->fn_strict &&
+				pertrans->initValueIsNull)
+				scratch->opcode = EEOP_AGG_PLAIN_TRANS_INIT_STRICT_BYVAL;
+			else if (fcinfo->flinfo->fn_strict)
+				scratch->opcode = EEOP_AGG_PLAIN_TRANS_STRICT_BYVAL;
+			else
+				scratch->opcode = EEOP_AGG_PLAIN_TRANS_BYVAL;
+		}
+		else
+		{
+			if (fcinfo->flinfo->fn_strict &&
+				pertrans->initValueIsNull)
+				scratch->opcode = EEOP_AGG_PLAIN_TRANS_INIT_STRICT_BYREF;
+			else if (fcinfo->flinfo->fn_strict)
+				scratch->opcode = EEOP_AGG_PLAIN_TRANS_STRICT_BYREF;
+			else
+				scratch->opcode = EEOP_AGG_PLAIN_TRANS_BYREF;
+		}
 	}
-
-	/* invoke appropriate transition implementation */
-	if (pertrans->numSortCols == 0 && pertrans->transtypeByVal)
-		scratch->opcode = EEOP_AGG_PLAIN_TRANS_BYVAL;
-	else if (pertrans->numSortCols == 0)
-		scratch->opcode = EEOP_AGG_PLAIN_TRANS;
 	else if (pertrans->numInputs == 1)
 		scratch->opcode = EEOP_AGG_ORDERED_TRANS_DATUM;
 	else
@@ -3296,22 +3303,6 @@ ExecBuildAggTransCall(ExprState *state, AggState *aggstate,
 	scratch->d.agg_trans.transno = transno;
 	scratch->d.agg_trans.aggcontext = aggcontext;
 	ExprEvalPushStep(state, scratch);
-
-	/* adjust jumps so they jump till after transition invocation */
-	if (adjust_init_jumpnull != -1)
-	{
-		ExprEvalStep *as = &state->steps[adjust_init_jumpnull];
-
-		Assert(as->d.agg_init_trans.jumpnull == -1);
-		as->d.agg_init_trans.jumpnull = state->steps_len;
-	}
-	if (adjust_strict_jumpnull != -1)
-	{
-		ExprEvalStep *as = &state->steps[adjust_strict_jumpnull];
-
-		Assert(as->d.agg_strict_trans_check.jumpnull == -1);
-		as->d.agg_strict_trans_check.jumpnull = state->steps_len;
-	}
 }
 
 /*