Hi,
Since
commit a7de3dc5c346e07e0439275982569996e645b3c2
Author: Robert Haas <rhaas@postgresql.org>
Date: 2016-01-20 13:46:50 -0500
Support multi-stage aggregation. Aggregate nodes now have two new modes: a "partial" mode where they output
theunfinalized transition state, and a "finalize" mode where they accept unfinalized transition states rather than
individual values as input. These new modes are not used anywhere yet, but they will be necessary for parallel
aggregation. The infrastructure also figures to be useful for cases where we want to aggregate local data and remote
data via the FDW interface, and want to bring back partial aggregates from the remote side that can then be combined
withlocally generated partial aggregates to produce the final value. It may also be useful even when neither FDWs
norparallelism are in play, as explained in the comments in nodeAgg.c. David Rowley and Simon Riggs, reviewed by
KaiGaiKohei, Heikki Linnakangas, Haribabu Kommi, and me.
there's both advance_transition_function and advance_combine_function,
fulfilling closely related duties.
While working on that code (making transition and combine functions go
through expression evaluation, so they can be JITed), I noticed a small
difference in behaviour beteween the two:
The plain transition case contains: if (pergroupstate->transValueIsNull) { /* * Don't call a
strictfunction with NULL inputs. Note it is * possible to get here despite the above tests, if the transfn is
* strict *and* returned a NULL on a prior cycle. If that happens * we will propagate the NULL all the way
tothe end. */ return; }
how come similar logic is not present for combine functions? I don't see
any checks preventing a combinefunc from returning NULL, nor do I see
https://www.postgresql.org/docs/devel/static/sql-createaggregate.html
spell out a requirement that that not be the case.
Greetings,
Andres Freund
