Speedup ScalarArrayOpExpr evaluation

ScalarArrayOpExprs with "useOr=true" and a set of Consts on the righthand
side have traditionally been evaluated by using a linear search over the
array.  When these arrays contain large numbers of elements then this
linear search could become a significant part of execution time.

Here we add a new method of evaluating ScalarArrayOpExpr expressions to
allow them to be evaluated by first building a hash table containing each
element, then on subsequent evaluations, we just probe that hash table to
determine if there is a match.

The planner is in charge of determining when this optimization is possible
and it enables it by setting hashfuncid in the ScalarArrayOpExpr.  The
executor will only perform the hash table evaluation when the hashfuncid
is set.

This means that not all cases are optimized. For example CHECK constraints
containing an IN clause won't go through the planner, so won't get the
hashfuncid set.  We could maybe do something about that at some later
date.  The reason we're not doing it now is from fear that we may slow
down cases where the expression is evaluated only once.  Those cases can
be common, for example, a single row INSERT to a table with a CHECK
constraint containing an IN clause.

In the planner, we enable this when there are suitable hash functions for
the ScalarArrayOpExpr's operator and only when there is at least
MIN_ARRAY_SIZE_FOR_HASHED_SAOP elements in the array.  The threshold is
currently set to 9.

Author: James Coleman, David Rowley
Reviewed-by: David Rowley, Tomas Vondra, Heikki Linnakangas
Discussion: https://postgr.es/m/CAAaqYe8x62+=wn0zvNKCj55tPpg-JBHzhZFFc6ANovdqFw7-dA@mail.gmail.com

src/test/regress/sql/expressions.sql

@@ -65,3 +65,88 @@ select count(*) from date_tbl
   where f1 not between symmetric '1997-01-01' and '1998-01-01';
 select count(*) from date_tbl
   where f1 not between symmetric '1997-01-01' and '1998-01-01';
+
+--
+-- Tests for ScalarArrayOpExpr with a hashfn
+--
+
+-- create a stable function so that the tests below are not
+-- evaluated using the planner's constant folding.
+begin;
+
+create function return_int_input(int) returns int as $$
+begin
+	return $1;
+end;
+$$ language plpgsql stable;
+
+create function return_text_input(text) returns text as $$
+begin
+	return $1;
+end;
+$$ language plpgsql stable;
+
+select return_int_input(1) in (10, 9, 2, 8, 3, 7, 4, 6, 5, 1);
+select return_int_input(1) in (10, 9, 2, 8, 3, 7, 4, 6, 5, null);
+select return_int_input(1) in (null, null, null, null, null, null, null, null, null, null, null);
+select return_int_input(1) in (10, 9, 2, 8, 3, 7, 4, 6, 5, 1, null);
+select return_int_input(null::int) in (10, 9, 2, 8, 3, 7, 4, 6, 5, 1);
+select return_int_input(null::int) in (10, 9, 2, 8, 3, 7, 4, 6, 5, null);
+select return_text_input('a') in ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j');
+
+rollback;
+
+-- Test with non-strict equality function.
+-- We need to create our own type for this.
+
+begin;
+
+create type myint;
+create function myintin(cstring) returns myint strict immutable language
+  internal as 'int4in';
+create function myintout(myint) returns cstring strict immutable language
+  internal as 'int4out';
+create function myinthash(myint) returns integer strict immutable language
+  internal as 'hashint4';
+
+create type myint (input = myintin, output = myintout, like = int4);
+
+create cast (int4 as myint) without function;
+create cast (myint as int4) without function;
+
+create function myinteq(myint, myint) returns bool as $$
+begin
+  if $1 is null and $2 is null then
+    return true;
+  else
+    return $1::int = $2::int;
+  end if;
+end;
+$$ language plpgsql immutable;
+
+create operator = (
+  leftarg    = myint,
+  rightarg   = myint,
+  commutator = =,
+  negator    = <>,
+  procedure  = myinteq,
+  restrict   = eqsel,
+  join       = eqjoinsel,
+  merges
+);
+
+create operator class myint_ops
+default for type myint using hash as
+  operator    1   =  (myint, myint),
+  function    1   myinthash(myint);
+
+create table inttest (a myint);
+insert into inttest values(1::myint),(null);
+
+-- try an array with enough elements to cause hashing
+select * from inttest where a in (1::myint,2::myint,3::myint,4::myint,5::myint,6::myint,7::myint,8::myint,9::myint, null);
+-- ensure the result matched with the non-hashed version.  We simply remove
+-- some array elements so that we don't reach the hashing threshold.
+select * from inttest where a in (1::myint,2::myint,3::myint,4::myint,5::myint, null);
+
+rollback;