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Fix foreign-key selectivity estimation in the presence of constants.

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get_foreign_key_join_selectivity() looks for join clauses that equate
the two sides of the FK constraint.  However, if we have a query like
"WHERE fktab.a = pktab.a and fktab.a = 1", it won't find any such join
clause, because equivclass.c replaces the given clauses with "fktab.a
= 1 and pktab.a = 1", which can be enforced at the scan level, leaving
nothing to be done for column "a" at the join level.

We can fix that expectation without much trouble, but then a new problem
arises: applying the foreign-key-based selectivity rule produces a
rowcount underestimate, because we're effectively double-counting the
selectivity of the "fktab.a = 1" clause.  So we have to cancel that
selectivity out of the estimate.

To fix, refactor process_implied_equality() so that it can pass back the
new RestrictInfo to its callers in equivclass.c, allowing the generated
"fktab.a = 1" clause to be saved in the EquivalenceClass's ec_derives
list.  Then it's not much trouble to dig out the relevant RestrictInfo
when we need to adjust an FK selectivity estimate.  (While at it, we
can also remove the expensive use of initialize_mergeclause_eclasses()
to set up the new RestrictInfo's left_ec and right_ec pointers.
The equivclass.c code can set those basically for free.)

This seems like clearly a bug fix, but I'm hesitant to back-patch it,
first because there's some API/ABI risk for extensions and second because
we're usually loath to destabilize plan choices in stable branches.

Per report from Sigrid Ehrenreich.

Discussion: https://postgr.es/m/1019549.1603770457@sss.pgh.pa.us
Discussion: https://postgr.es/m/AM6PR02MB5287A0ADD936C1FA80973E72AB190@AM6PR02MB5287.eurprd02.prod.outlook.com
This commit is contained in:
Tom Lane
2020-10-28 11:15:47 -04:00
parent ce7f772c5e
commit ad1c36b070
10 changed files with 366 additions and 103 deletions
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3
src/include/nodes/pathnodes.h
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@ -889,10 +889,13 @@ typedef struct ForeignKeyOptInfo
/* Derived info about whether FK's equality conditions match the query: */
int nmatched_ec; /* # of FK cols matched by ECs */
int nconst_ec; /* # of these ECs that are ec_has_const */
int nmatched_rcols; /* # of FK cols matched by non-EC rinfos */
int nmatched_ri; /* total # of non-EC rinfos matched to FK */
/* Pointer to eclass matching each column's condition, if there is one */
struct EquivalenceClass *eclass[INDEX_MAX_KEYS];
/* Pointer to eclass member for the referencing Var, if there is one */
struct EquivalenceMember *fk_eclass_member[INDEX_MAX_KEYS];
/* List of non-EC RestrictInfos matching each column's condition */
List *rinfos[INDEX_MAX_KEYS];
} ForeignKeyOptInfo;