Free list same_input_transnos in preprocess_aggref

Zhang Mingli <zmlpostgres@gmail.com> writes:

In preprocess_aggref(), list same_input_transnos is used to track compatible transnos.
Free it if we don’t need it anymore.

Very little of the planner bothers with freeing small allocations
like that. Can you demonstrate a case where this would actually
make a meaningful difference?

regards, tom lane

Regards,
Zhang Mingli
On Sep 19, 2022, 23:14 +0800, Tom Lane <tgl@sss.pgh.pa.us>, wrote:

Very little of the planner bothers with freeing small allocations
like that.

I think so too, as said, not sure if it worths.

Can you demonstrate a case where this would actually
make a meaningful difference?

Offhand, an example may help a little:

create table t1(id int);
explain select max(id), min(id), sum(id), count(id), avg(id) from t1;

 Modify codes to test:

@@ -139,6 +139,7 @@ preprocess_aggref(Aggref *aggref, PlannerInfo *root)
 int16 transtypeLen;
 Oid inputTypes[FUNC_MAX_ARGS];
 int numArguments;
+ static size_t accumulate_list_size = 0;

 Assert(aggref->agglevelsup == 0);

@@ -265,7 +266,7 @@ preprocess_aggref(Aggref *aggref, PlannerInfo *root)
 aggserialfn, aggdeserialfn,
 initValue, initValueIsNull,
 same_input_transnos);
- list_free(same_input_transnos);
+ accumulate_list_size += sizeof(int) * list_length(same_input_transnos);

Gdb and print accumulate_list_size for each iteration:

SaveBytes = Sum results of accumulate_list_size: 32(4+4+8+8), as we have 5 aggs in sql.

If there were N sets of that aggs (more columns as id, with above aggs ), the bytes will be N*SaveBytes.

Seems we don’t have so many agg functions that could share the same trans function, Does it worth?

Regards,
Zhang Mingli
On Sep 20, 2022, 00:27 +0800, Zhang Mingli <zmlpostgres@gmail.com>, wrote:

SaveBytes = Sum results of accumulate_list_size: 32(4+4+8+8), as we have 5 aggs in sql

Correction: SaveBytes = Sum results of accumulate_list_size: 24(4+4+8+8),

Zhang Mingli <zmlpostgres@gmail.com> writes:

Correction: SaveBytes = Sum results of accumulate_list_size: 24(4+4+8+8),

What I did was to stick in

elog(LOG, "leaking list of length %d", list_length(same_input_transnos));

at the end of preprocess_aggref. What I see on your five-aggregate
example is

2022-11-06 14:59:25.666 EST [3046253] LOG: leaking list of length 0
2022-11-06 14:59:25.666 EST [3046253] STATEMENT: explain select max(id), min(id), sum(id), count(id), avg(id) from t1;
2022-11-06 14:59:25.666 EST [3046253] LOG: leaking list of length 1
2022-11-06 14:59:25.666 EST [3046253] STATEMENT: explain select max(id), min(id), sum(id), count(id), avg(id) from t1;
2022-11-06 14:59:25.666 EST [3046253] LOG: leaking list of length 0
2022-11-06 14:59:25.666 EST [3046253] STATEMENT: explain select max(id), min(id), sum(id), count(id), avg(id) from t1;
2022-11-06 14:59:25.666 EST [3046253] LOG: leaking list of length 1
2022-11-06 14:59:25.666 EST [3046253] STATEMENT: explain select max(id), min(id), sum(id), count(id), avg(id) from t1;
2022-11-06 14:59:25.666 EST [3046253] LOG: leaking list of length 0
2022-11-06 14:59:25.666 EST [3046253] STATEMENT: explain select max(id), min(id), sum(id), count(id), avg(id) from t1;

The NIL lists are of course occupying no storage. The two one-element
lists are absolutely, completely negligible in the context of planning
any nontrivial statement. Even the aggtransinfos list that is the
primary output of preprocess_aggref will dwarf that; and we leak
similarly small data structures in probably many hundred places in
the planner.

I went a bit further and ran the core regression tests, then aggregated
the results:

$ grep 'leaking list' postmaster.log | sed 's/.*] //' | sort | uniq -c
4516 LOG: leaking list of length 0
95 LOG: leaking list of length 1
15 LOG: leaking list of length 2

You can quibble of course about how representative the regression tests
are, but there's sure no evidence at all here that we'd be saving
anything measurable.

If anything, I'd be inclined to get rid of the

list_free(*same_input_transnos);

in find_compatible_agg, because it seems like a waste of code on
the same grounds. Instrumenting that in the same way, I find
that it's not reached at all in your example, while the
regression tests give

49 LOG: freeing list of length 0
2 LOG: freeing list of length 1

regards, tom lane

HI,

On Nov 7, 2022, 04:12 +0800, Tom Lane <tgl@sss.pgh.pa.us>, wrote:

The NIL lists are of course occupying no storage. The two one-element
lists are absolutely, completely negligible in the context of planning
any nontrivial statement. Even the aggtransinfos list that is the
primary output of preprocess_aggref will dwarf that; and we leak
similarly small data structures in probably many hundred places in
the planner.

I went a bit further and ran the core regression tests, then aggregated
the results:

$ grep 'leaking list' postmaster.log | sed 's/.*] //' | sort | uniq -c
4516 LOG: leaking list of length 0
95 LOG: leaking list of length 1
15 LOG: leaking list of length 2

You can quibble of course about how representative the regression tests
are, but there's sure no evidence at all here that we'd be saving
anything measurable.

If anything, I'd be inclined to get rid of the

list_free(*same_input_transnos);

in find_compatible_agg, because it seems like a waste of code on
the same grounds. Instrumenting that in the same way, I find
that it's not reached at all in your example, while the
regression tests give

49 LOG: freeing list of length 0
2 LOG: freeing list of length 1

Thanks for the investigation.
Yeah, this patch is negligible. I’ll withdraw it in CF.

Regards,
Zhang Mingli