Re: Tid scan improvements
| От | Edmund Horner |
|---|---|
| Тема | Re: Tid scan improvements |
| Дата | |
| Msg-id | CAMyN-kDvdcpUjTD5nSxhLV8+77+-qQ-Ug8UJcux-HCoK9zqJJA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Tid scan improvements (Tom Lane <tgl@sss.pgh.pa.us>) |
| Ответы |
Re: Tid scan improvements
|
| Список | pgsql-hackers |
On Sat, 22 Dec 2018 at 07:10, Tom Lane <tgl@sss.pgh.pa.us> wrote:
BTW, with respect to this bit in 0001:
@@ -1795,6 +1847,15 @@ nulltestsel(PlannerInfo *root, NullTestType nulltesttype, Node *arg,
return (Selectivity) 0; /* keep compiler quiet */
}
}
+ else if (vardata.var && IsA(vardata.var, Var) &&
+ ((Var *) vardata.var)->varattno == SelfItemPointerAttributeNumber)
+ {
+ /*
+ * There are no stats for system columns, but we know CTID is never
+ * NULL.
+ */
+ selec = (nulltesttype == IS_NULL) ? 0.0 : 1.0;
+ }
else
{
/*
I'm not entirely sure why you're bothering; surely nulltestsel is
unrelated to what this patch is about? And would anybody really
write "WHERE ctid IS NULL"?
I found that it made a difference with selectivity of range comparisons, because clauselist_selectivity tries to correct for it (clausesel.c:274):
s2 = rqlist->hibound + rqlist->lobound - 1.0
/* Adjust for double-exclusion of NULLs */
s2 += nulltestsel(root, IS_NULL, rqlist->var,
varRelid, jointype, sjinfo);
It was adding DEFAULT_UNK_SEL = 0.005 to the selectivity, which (while not major) did make the selectivity less accurate.
However, if we do think it's worth adding code to cover this case,
I wouldn't make it specific to CTID. *All* system columns can be
assumed not null, see heap_getsysattr().
I guess we could have a standalone patch to add this for all system columns?
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