Improve planner to drop constant-NULL inputs of AND/OR where it's legal.

In general we can't discard constant-NULL inputs, since they could change
the result of the AND/OR to be NULL.  But at top level of WHERE, we do not
need to distinguish a NULL result from a FALSE result, so it's okay to
treat NULL as FALSE and then simplify AND/OR accordingly.

This is a very ancient oversight, but in 9.2 and later it can lead to
failure to optimize queries that previous releases did optimize, as a
result of more aggressive parameter substitution rules making it possible
to reduce more subexpressions to NULL constants.  This is the root cause of
bug #10171 from Arnold Scheffler.  We could alternatively have fixed that
by teaching orclauses.c to ignore constant-NULL OR arms, but it seems
better to get rid of them globally.

I resisted the temptation to back-patch this change into all active
branches, but it seems appropriate to back-patch as far as 9.2 so that
there will not be performance regressions of the kind shown in this bug.

diff --git a/src/backend/optimizer/prep/prepqual.c b/src/backend/optimizer/prep/prepqual.c
index e4ce32830df3d3d1ffbd1dde099007d1b81e667e..812fbaddba9e9b2dada2d98fc006aa23129751e1 100644 (file)
--- a/src/backend/optimizer/prep/prepqual.c
+++ b/src/backend/optimizer/prep/prepqual.c
@@ -293,7 +293,7 @@ canonicalize_qual(Expr *qual)
    /*
     * Pull up redundant subclauses in OR-of-AND trees.  We do this only
     * within the top-level AND/OR structure; there's no point in looking
-    * deeper.
+    * deeper.  Also remove any NULL constants in the top-level structure.
     */
    newqual = find_duplicate_ors(qual);
 
@@ -393,6 +393,13 @@ pull_ors(List *orlist)
  *   OR clauses to which the inverse OR distributive law might apply.
  *   Only the top-level AND/OR structure is searched.
  *
+ * While at it, we remove any NULL constants within the top-level AND/OR
+ * structure, eg "x OR NULL::boolean" is reduced to "x".  In general that
+ * would change the result, so eval_const_expressions can't do it; but at
+ * top level of WHERE, we don't need to distinguish between FALSE and NULL
+ * results, so it's valid to treat NULL::boolean the same as FALSE and then
+ * simplify AND/OR accordingly.
+ *
  * Returns the modified qualification. AND/OR flatness is preserved.
  */
 static Expr *
@@ -405,12 +412,30 @@ find_duplicate_ors(Expr *qual)
 
        /* Recurse */
        foreach(temp, ((BoolExpr *) qual)->args)
-           orlist = lappend(orlist, find_duplicate_ors(lfirst(temp)));
+       {
+           Expr       *arg = (Expr *) lfirst(temp);
 
-       /*
-        * Don't need pull_ors() since this routine will never introduce an OR
-        * where there wasn't one before.
-        */
+           arg = find_duplicate_ors(arg);
+
+           /* Get rid of any constant inputs */
+           if (arg && IsA(arg, Const))
+           {
+               Const      *carg = (Const *) arg;
+
+               /* Drop constant FALSE or NULL */
+               if (carg->constisnull || !DatumGetBool(carg->constvalue))
+                   continue;
+               /* constant TRUE, so OR reduces to TRUE */
+               return arg;
+           }
+
+           orlist = lappend(orlist, arg);
+       }
+
+       /* Flatten any ORs pulled up to just below here */
+       orlist = pull_ors(orlist);
+
+       /* Now we can look for duplicate ORs */
        return process_duplicate_ors(orlist);
    }
    else if (and_clause((Node *) qual))
@@ -420,10 +445,38 @@ find_duplicate_ors(Expr *qual)
 
        /* Recurse */
        foreach(temp, ((BoolExpr *) qual)->args)
-           andlist = lappend(andlist, find_duplicate_ors(lfirst(temp)));
+       {
+           Expr       *arg = (Expr *) lfirst(temp);
+
+           arg = find_duplicate_ors(arg);
+
+           /* Get rid of any constant inputs */
+           if (arg && IsA(arg, Const))
+           {
+               Const      *carg = (Const *) arg;
+
+               /* Drop constant TRUE */
+               if (!carg->constisnull && DatumGetBool(carg->constvalue))
+                   continue;
+               /* constant FALSE or NULL, so AND reduces to FALSE */
+               return (Expr *) makeBoolConst(false, false);
+           }
+
+           andlist = lappend(andlist, arg);
+       }
+
        /* Flatten any ANDs introduced just below here */
        andlist = pull_ands(andlist);
-       /* The AND list can't get shorter, so result is always an AND */
+
+       /* AND of no inputs reduces to TRUE */
+       if (andlist == NIL)
+           return (Expr *) makeBoolConst(true, false);
+
+       /* Single-expression AND just reduces to that expression */
+       if (list_length(andlist) == 1)
+           return (Expr *) linitial(andlist);
+
+       /* Else we still need an AND node */
        return make_andclause(andlist);
    }
    else
@@ -447,11 +500,13 @@ process_duplicate_ors(List *orlist)
    List       *neworlist;
    ListCell   *temp;
 
+   /* OR of no inputs reduces to FALSE */
    if (orlist == NIL)
-       return NULL;            /* probably can't happen */
-   if (list_length(orlist) == 1)       /* single-expression OR (can this
-                                        * happen?) */
-       return linitial(orlist);
+       return (Expr *) makeBoolConst(false, false);
+
+   /* Single-expression OR just reduces to that expression */
+   if (list_length(orlist) == 1)
+       return (Expr *) linitial(orlist);
 
    /*
     * Choose the shortest AND clause as the reference list --- obviously, any

diff --git a/src/test/regress/expected/create_index.out b/src/test/regress/expected/create_index.out
index f13b4f8a08d15b7de9e66b272f4377d9f94fcdd8..f6f551632117d8f3dfdde378e8332681102dd6a1 100644 (file)
--- a/src/test/regress/expected/create_index.out
+++ b/src/test/regress/expected/create_index.out
@@ -2771,3 +2771,14 @@ ORDER BY thousand;
         1 |     1001
 (2 rows)
 
+--
+-- Check elimination of constant-NULL subexpressions
+--
+explain (costs off)
+  select * from tenk1 where (thousand, tenthous) in ((1,1001), (null,null));
+                      QUERY PLAN                      
+------------------------------------------------------
+ Index Scan using tenk1_thous_tenthous on tenk1
+   Index Cond: ((thousand = 1) AND (tenthous = 1001))
+(2 rows)
+

diff --git a/src/test/regress/sql/create_index.sql b/src/test/regress/sql/create_index.sql
index cd5c58d468cf8745caa1d00a1453d1daabb496fd..d4d24ef82b413a204cae7682c84c69dcdc3dd8fd 100644 (file)
--- a/src/test/regress/sql/create_index.sql
+++ b/src/test/regress/sql/create_index.sql
@@ -931,3 +931,10 @@ ORDER BY thousand;
 SELECT thousand, tenthous FROM tenk1
 WHERE thousand < 2 AND tenthous IN (1001,3000)
 ORDER BY thousand;
+
+--
+-- Check elimination of constant-NULL subexpressions
+--
+
+explain (costs off)
+  select * from tenk1 where (thousand, tenthous) in ((1,1001), (null,null));