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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

31
contrib/intarray/_int_selfuncs.c
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@ -136,11 +136,7 @@ _int_matchsel(PG_FUNCTION_ARGS)
int nmcelems = 0;
float4 minfreq = 0.0;
float4 nullfrac = 0.0;
Form_pg_statistic stats;
Datum *values = NULL;
int nvalues = 0;
float4 *numbers = NULL;
int nnumbers = 0;
AttStatsSlot sslot;
/*
* If expression is not "variable @@ something" or "something @@ variable"
@ -193,6 +189,8 @@ _int_matchsel(PG_FUNCTION_ARGS)
*/
if (HeapTupleIsValid(vardata.statsTuple))
{
Form_pg_statistic stats;
stats = (Form_pg_statistic) GETSTRUCT(vardata.statsTuple);
nullfrac = stats->stanullfrac;
@ -200,29 +198,30 @@ _int_matchsel(PG_FUNCTION_ARGS)
* For an int4 array, the default array type analyze function will
* collect a Most Common Elements list, which is an array of int4s.
*/
if (get_attstatsslot(vardata.statsTuple,
INT4OID, -1,
if (get_attstatsslot(&sslot, vardata.statsTuple,
STATISTIC_KIND_MCELEM, InvalidOid,
NULL,
&values, &nvalues,
&numbers, &nnumbers))
ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
{
Assert(sslot.valuetype == INT4OID);
/*
* There should be three more Numbers than Values, because the
* last three (for intarray) cells are taken for minimal, maximal
* and nulls frequency. Punt if not.
*/
if (nnumbers == nvalues + 3)
if (sslot.nnumbers == sslot.nvalues + 3)
{
/* Grab the lowest frequency. */
minfreq = numbers[nnumbers - (nnumbers - nvalues)];
minfreq = sslot.numbers[sslot.nnumbers - (sslot.nnumbers - sslot.nvalues)];
mcelems = values;
mcefreqs = numbers;
nmcelems = nvalues;
mcelems = sslot.values;
mcefreqs = sslot.numbers;
nmcelems = sslot.nvalues;
}
}
}
else
memset(&sslot, 0, sizeof(sslot));
/* Process the logical expression in the query, using the stats */
selec = int_query_opr_selec(GETQUERY(query) + query->size - 1,
@ -231,7 +230,7 @@ _int_matchsel(PG_FUNCTION_ARGS)
/* MCE stats count only non-null rows, so adjust for null rows. */
selec *= (1.0 - nullfrac);
free_attstatsslot(INT4OID, values, nvalues, numbers, nnumbers);
free_attstatsslot(&sslot);
ReleaseVariableStats(vardata);
CLAMP_PROBABILITY(selec);