Re: Hash support for grouping sets

> On Mar 8, 2017, at 9:40 AM, Andrew Gierth <andrew@tao11.riddles.org.uk> wrote:
> 
>>>>>> "Mark" == Mark Dilger <hornschnorter@gmail.com> writes:
> 
> Mark> Hi Andrew,
> 
> Mark> Reviewing the patch a bit more, I find it hard to understand the
> Mark> comment about passing -1 as a flag for finalize_aggregates.  Any
> Mark> chance you can spend a bit more time word-smithing that code
> Mark> comment?
> 
> Sure.
> 
> How does this look for wording (I'll incorporate it into an updated
> patch later):

Much better.  Thanks!

> /*
> * Compute the final value of all aggregates for one group.
> *
> * This function handles only one grouping set at a time.  But in the hash
> * case, it's the caller's responsibility to have selected the set already, and
> * we pass in -1 as currentSet here to flag that; this also changes how we
> * handle the indexing into AggStatePerGroup as explained below.
> *
> * Results are stored in the output econtext aggvalues/aggnulls.
> */
> static void
> finalize_aggregates(AggState *aggstate,
> 					AggStatePerAgg peraggs,
> 					AggStatePerGroup pergroup,
> 					int currentSet)
> {
> 	ExprContext *econtext = aggstate->ss.ps.ps_ExprContext;
> 	Datum	   *aggvalues = econtext->ecxt_aggvalues;
> 	bool	   *aggnulls = econtext->ecxt_aggnulls;
> 	int			aggno;
> 	int			transno;
> 
> 	/*
> 	 * If currentSet >= 0, then we're doing sorted grouping, and pergroup is an
> 	 * array of size numTrans*numSets which we have to index into using
> 	 * currentSet in addition to transno. The caller may not have selected the
> 	 * set, so we do that.
> 	 *
> 	 * If currentSet < 0, then we're doing hashed grouping, and pergroup is an
> 	 * array of only numTrans items (since for hashed grouping, each grouping
> 	 * set is in a separate hashtable).  We rely on the caller having done
> 	 * select_current_set, and we fudge currentSet to 0 in order to make the
> 	 * same indexing calculations work as for the grouping case.
> 	 */
> 	if (currentSet >= 0)
> 		select_current_set(aggstate, currentSet, false);
> 	else
> 		currentSet = 0;
> 
> 
> -- 
> Andrew (irc:RhodiumToad)