Don't read MCV stats needlessly in eqjoinsel().

eqjoinsel() currently makes use of MCV stats only when we have such
stats for both sides of the clause.  As coded, though, it would
fetch those stats even when they're present for just one side.
This can be a bit expensive with high statistics targets, leading
to wasted effort in common cases such as joining a unique column
to a non-unique column.  So it seems worth the trouble to do a quick
pre-check to confirm that both sides have MCVs before fetching either.

Also, tweak the API spec for get_attstatsslot() to document the
method we're using here.

David Geier, Tomas Vondra, Tom Lane

Discussion: https://postgr.es/m/b9846ca0-5f1c-9b26-5881-aad3f42b07f0@gmail.com

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index d597b7e81fc189e038907f4d3120137e53d320a9..e0aeaa6909250456aaef50a892a742410f50fe08 100644 (file)
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -2261,6 +2261,7 @@ eqjoinsel(PG_FUNCTION_ARGS)
    Form_pg_statistic stats2 = NULL;
    bool        have_mcvs1 = false;
    bool        have_mcvs2 = false;
+   bool        get_mcv_stats;
    bool        join_is_reversed;
    RelOptInfo *inner_rel;
 
@@ -2275,11 +2276,25 @@ eqjoinsel(PG_FUNCTION_ARGS)
    memset(&sslot1, 0, sizeof(sslot1));
    memset(&sslot2, 0, sizeof(sslot2));
 
+   /*
+    * There is no use in fetching one side's MCVs if we lack MCVs for the
+    * other side, so do a quick check to verify that both stats exist.
+    */
+   get_mcv_stats = (HeapTupleIsValid(vardata1.statsTuple) &&
+                    HeapTupleIsValid(vardata2.statsTuple) &&
+                    get_attstatsslot(&sslot1, vardata1.statsTuple,
+                                     STATISTIC_KIND_MCV, InvalidOid,
+                                     0) &&
+                    get_attstatsslot(&sslot2, vardata2.statsTuple,
+                                     STATISTIC_KIND_MCV, InvalidOid,
+                                     0));
+
    if (HeapTupleIsValid(vardata1.statsTuple))
    {
        /* note we allow use of nullfrac regardless of security check */
        stats1 = (Form_pg_statistic) GETSTRUCT(vardata1.statsTuple);
-       if (statistic_proc_security_check(&vardata1, opfuncoid))
+       if (get_mcv_stats &&
+           statistic_proc_security_check(&vardata1, opfuncoid))
            have_mcvs1 = get_attstatsslot(&sslot1, vardata1.statsTuple,
                                          STATISTIC_KIND_MCV, InvalidOid,
                                          ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS);
@@ -2289,7 +2304,8 @@ eqjoinsel(PG_FUNCTION_ARGS)
    {
        /* note we allow use of nullfrac regardless of security check */
        stats2 = (Form_pg_statistic) GETSTRUCT(vardata2.statsTuple);
-       if (statistic_proc_security_check(&vardata2, opfuncoid))
+       if (get_mcv_stats &&
+           statistic_proc_security_check(&vardata2, opfuncoid))
            have_mcvs2 = get_attstatsslot(&sslot2, vardata2.statsTuple,
                                          STATISTIC_KIND_MCV, InvalidOid,
                                          ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS);

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index a16a63f4957b525cfa9c1d0d694e91401ae9c100..94ca8e12303def4b5726c2a6d6d56ff8edccbab3 100644 (file)
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -3183,6 +3183,10 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
  *
  * If it's desirable to call free_attstatsslot when get_attstatsslot might
  * not have been called, memset'ing sslot to zeroes will allow that.
+ *
+ * Passing flags=0 can be useful to quickly check if the requested slot type
+ * exists.  In this case no arrays are extracted, so free_attstatsslot need
+ * not be called.
  */
 bool
 get_attstatsslot(AttStatsSlot *sslot, HeapTuple statstuple,