Re: BitmapHeapScan streaming read user and prelim refactoring

On 2024-May-14, Tomas Vondra wrote:

> On 5/14/24 19:42, Melanie Plageman wrote:
> 
> >>> +SET enable_indexonlyscan = off;
> >>> +set enable_indexscan = off;
> >>> +SET enable_seqscan = off;
> >>
> >> Nit: adjusting the casing of the second SET here.
> > 
> > I've fixed this. I've also set enable_material off as I mentioned I
> > might in my earlier mail.
>
> I'm not sure this (setting more and more GUCs to prevent hypothetical
> plan changes) is a good practice. Because how do you know the plan does
> not change for some other unexpected reason, possibly in the future?

I wonder why it resets enable_indexscan at all.  I see that this query
first tries a seqscan, then if you disable that it tries an index only
scan, and if you disable that you get the expected bitmap indexscan.
But an indexscan doesn't seem to be in the cards.

> IMHO if the test requires a specific plan, it's better to do an actual
> "explain (rows off, costs off)" to check that.

That's already in the patch, right?

I do wonder how do we _know_ that the test is testing what it wants to
test:
                       QUERY PLAN                        
─────────────────────────────────────────────────────────
 Nested Loop Anti Join
   ->  Seq Scan on skip_fetch t1
   ->  Materialize
         ->  Bitmap Heap Scan on skip_fetch t2
               Recheck Cond: (a = 1)
               ->  Bitmap Index Scan on skip_fetch_a_idx
                     Index Cond: (a = 1)

Is it because of the shape of the index condition?  Maybe it's worth
explaining in the comments for the tests.

BTW, I was running the explain while desultorily enabling and disabling
these GUCs and hit this assertion failure:

#4  0x000055e6c72afe28 in ExceptionalCondition (conditionName=conditionName@entry=0x55e6c731a928
"scan->rs_empty_tuples_pending== 0", 
 
    fileName=fileName@entry=0x55e6c731a3b0
"../../../../../../../../../pgsql/source/master/src/backend/access/heap/heapam.c",lineNumber=lineNumber@entry=1219)
 
    at ../../../../../../../../../pgsql/source/master/src/backend/utils/error/assert.c:66
#5  0x000055e6c6e2e0c7 in heap_endscan (sscan=0x55e6c7b63e28) at
../../../../../../../../../pgsql/source/master/src/backend/access/heap/heapam.c:1219
#6  0x000055e6c6fb35a7 in ExecEndPlan (estate=0x55e6c7a7e9d0, planstate=<optimized out>) at
../../../../../../../../pgsql/source/master/src/backend/executor/execMain.c:1485
#7  standard_ExecutorEnd (queryDesc=0x55e6c7a736b8) at
../../../../../../../../pgsql/source/master/src/backend/executor/execMain.c:501
#8  0x000055e6c6f4d9aa in ExplainOnePlan (plannedstmt=plannedstmt@entry=0x55e6c7a735a8, into=into@entry=0x0,
es=es@entry=0x55e6c7a448b8,
 
    queryString=queryString@entry=0x55e6c796c210 "EXPLAIN (analyze, verbose, COSTS OFF) SELECT t1.a FROM skip_fetch t1
LEFTJOIN skip_fetch t2 ON t2.a = 1 WHERE t2.a IS NULL;", params=params@entry=0x0, 
 
    queryEnv=queryEnv@entry=0x0, planduration=0x7ffe8a291848, bufusage=0x0, mem_counters=0x0) at
../../../../../../../../pgsql/source/master/src/backend/commands/explain.c:770
#9  0x000055e6c6f4e257 in standard_ExplainOneQuery (query=<optimized out>, cursorOptions=2048, into=0x0,
es=0x55e6c7a448b8,
 
    queryString=0x55e6c796c210 "EXPLAIN (analyze, verbose, COSTS OFF) SELECT t1.a FROM skip_fetch t1 LEFT JOIN
skip_fetcht2 ON t2.a = 1 WHERE t2.a IS NULL;", params=0x0, queryEnv=0x0)
 
    at ../../../../../../../../pgsql/source/master/src/backend/commands/explain.c:502

I couldn't reproduce it again, though -- and for sure I don't know what
it means.  All three GUCs are set false in the core.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Here's a general engineering tip: if the non-fun part is too complex for you
to figure out, that might indicate the fun part is too ambitious." (John Naylor)
https://postgr.es/m/CAFBsxsG4OWHBbSDM%3DsSeXrQGOtkPiOEOuME4yD7Ce41NtaAD9g%40mail.gmail.com