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Redesign get_attstatsslot()/free_attstatsslot() for more safety and speed.

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The mess cleaned up in commit da0759600 is clear evidence that it's a
bug hazard to expect the caller of get_attstatsslot()/free_attstatsslot()
to provide the correct type OID for the array elements in the slot.
Moreover, we weren't even getting any performance benefit from that,
since get_attstatsslot() was extracting the real type OID from the array
anyway.  So we ought to get rid of that requirement; indeed, it would
make more sense for get_attstatsslot() to pass back the type OID it found,
in case the caller isn't sure what to expect, which is likely in binary-
compatible-operator cases.

Another problem with the current implementation is that if the stats array
element type is pass-by-reference, we incur a palloc/memcpy/pfree cycle
for each element.  That seemed acceptable when the code was written because
we were targeting O(10) array sizes --- but these days, stats arrays are
almost always bigger than that, sometimes much bigger.  We can save a
significant number of cycles by doing one palloc/memcpy/pfree of the whole
array.  Indeed, in the now-probably-common case where the array is toasted,
that happens anyway so this method is basically free.  (Note: although the
catcache code will inline any out-of-line toasted values, it doesn't
decompress them.  At the other end of the size range, it doesn't expand
short-header datums either.  In either case, DatumGetArrayTypeP would have
to make a copy.  We do end up using an extra array copy step if the element
type is pass-by-value and the array length is neither small enough for a
short header nor large enough to have suffered compression.  But that
seems like a very acceptable price for winning in pass-by-ref cases.)

Hence, redesign to take these insights into account.  While at it,
convert to an API in which we fill a struct rather than passing a bunch
of pointers to individual output arguments.  That will make it less
painful if we ever want further expansion of what get_attstatsslot can
pass back.

It's certainly arguable that this is new development and not something to
push post-feature-freeze.  However, I view it as primarily bug-proofing
and therefore something that's better to have sooner not later.  Since
we aren't quite at beta phase yet, let's put it in.

Discussion: https://postgr.es/m/16364.1494520862@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2017-05-13 15:14:39 -04:00
parent 1848b73d45
commit 9aab83fc50
10 changed files with 434 additions and 599 deletions
Download Patch File Download Diff File Expand all files Collapse all files

25
src/backend/executor/nodeHash.c
View File

@ -1283,10 +1283,7 @@ static void
ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
{
HeapTupleData *statsTuple;
Datum *values;
int nvalues;
float4 *numbers;
int nnumbers;
AttStatsSlot sslot;
/* Do nothing if planner didn't identify the outer relation's join key */
if (!OidIsValid(node->skewTable))
@ -1305,19 +1302,17 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
if (!HeapTupleIsValid(statsTuple))
return;
if (get_attstatsslot(statsTuple, node->skewColType, node->skewColTypmod,
if (get_attstatsslot(&sslot, statsTuple,
STATISTIC_KIND_MCV, InvalidOid,
NULL,
&values, &nvalues,
&numbers, &nnumbers))
ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
{
double frac;
int nbuckets;
FmgrInfo *hashfunctions;
int i;
if (mcvsToUse > nvalues)
mcvsToUse = nvalues;
if (mcvsToUse > sslot.nvalues)
mcvsToUse = sslot.nvalues;
/*
* Calculate the expected fraction of outer relation that will
@ -1326,11 +1321,10 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
*/
frac = 0;
for (i = 0; i < mcvsToUse; i++)
frac += numbers[i];
frac += sslot.numbers[i];
if (frac < SKEW_MIN_OUTER_FRACTION)
{
free_attstatsslot(node->skewColType,
values, nvalues, numbers, nnumbers);
free_attstatsslot(&sslot);
ReleaseSysCache(statsTuple);
return;
}
@ -1392,7 +1386,7 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
int bucket;
hashvalue = DatumGetUInt32(FunctionCall1(&hashfunctions[0],
values[i]));
sslot.values[i]));
/*
* While we have not hit a hole in the hashtable and have not hit
@ -1426,8 +1420,7 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
hashtable->spacePeak = hashtable->spaceUsed;
}
free_attstatsslot(node->skewColType,
values, nvalues, numbers, nnumbers);
free_attstatsslot(&sslot);
}
ReleaseSysCache(statsTuple);