@@ -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?
