Re: [HACKERS] eval_const_expresisions and ScalarArrayOpExpr

Dmitry Dolgov <9erthalion6@gmail.com> writes:
> I tried to experiment a bit with this patch, hope it may be helpful.

Thanks for reviewing!

I took your idea of just running pgbench results, and adapted it to these
test cases:

select * from test where 1 = 1 or 1 = 2;

select * from test where 1 = 1 and 1 = 2;

select * from test where 1 in (1, 2);

select * from test where id in (1,2);

where the test table is just created with
    create table test (id int);
and contains no data.

For me, the first, second, and fourth cases are within 1% of the
same speed with or without the patch; some a bit slower, some a
bit faster, but really all of those results are barely above the
noise floor.  The third case is distinctly faster (~8%) with patch;
but since we're emitting a better plan, with no runtime WHERE test,
that's probably more about reduced executor overhead than anything else.

I did find one case where the patch makes things significantly slower:

select * from test where true is true
 and true is true
 and true is true
 and true is true
 ... (100 times altogether)

That's a full 25% slower with the patch.  Investigation confirms
that the extra cost can be blamed on using evaluate_expr() to
simplify BooleanTest nodes, in place of the specialized logic
that's there now.  I don't feel bad about the other places where
evaluate_expr() is used: either they were like that before, or
the case wasn't const-simplified at all, so that even with some
overhead we can be pretty sure life is better with the patch.
But there's room to argue that BooleanTest occurs often enough that
it's worth having bespoke logic to simplify it, if that logic isn't too
complicated which it really isn't.  So I'm inclined to undo the part
of the patch that turns BooleanTest processing into a generic case,
as per attached updated patch.  Otherwise I think we can figure that
performance isn't a problem.

> Speaking about the code, everything looks fine. As for me, some
> variables could be named in a more self-explanatory way (e.g
> `ece_evaluate_expr`, where `ece` is `eval_const_expresisions`, or
> `saop`, which is `ScalarArrayOp`), but it's minor.

I'm disinclined to change the usage of "saop" here --- that's already
in use in lots of code touching ScalarArrayOp, so even if we had a better
idea, this patch isn't the place to change it.  I'd be fine with changing
the "ece_" prefix, but I don't want to spell out eval_const_expressions
altogether; do you have a different abbreviation suggestion?

> I wonder if it makes sense in this patch also deal with the following
> situation?

> explain analyze select * from test where 1 in (select 1);

No; lots of people depend on the fact that a sub-select is an optimization
fence in that way.  If we were going to change that it'd need to be
carefully thought through, and I don't think it should happen as a side
effect of what's mostly a refactoring patch.  The code changes wouldn't
be anywhere near this patch, either ...

            regards, tom lane

diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index 9ca384d..cba40b2 100644
*** a/src/backend/optimizer/util/clauses.c
--- b/src/backend/optimizer/util/clauses.c
*************** static List *find_nonnullable_vars_walke
*** 115,120 ****
--- 115,123 ----
  static bool is_strict_saop(ScalarArrayOpExpr *expr, bool falseOK);
  static Node *eval_const_expressions_mutator(Node *node,
                                 eval_const_expressions_context *context);
+ static bool contain_non_const_walker(Node *node, void *context);
+ static bool ece_function_is_safe(Oid funcid,
+                      eval_const_expressions_context *context);
  static List *simplify_or_arguments(List *args,
                        eval_const_expressions_context *context,
                        bool *haveNull, bool *forceTrue);
*************** estimate_expression_value(PlannerInfo *r
*** 2502,2507 ****
--- 2505,2541 ----
      return eval_const_expressions_mutator(node, &context);
  }

+ /*
+  * The generic case in eval_const_expressions_mutator is to recurse using
+  * expression_tree_mutator, which will copy the given node unchanged but
+  * const-simplify its arguments (if any) as far as possible.  If the node
+  * itself does immutable processing, and each of its arguments were reduced
+  * to a Const, we can then reduce it to a Const using evaluate_expr.  (Some
+  * node types need more complicated logic; for example, a CASE expression
+  * might be reducible to a constant even if not all its subtrees are.)
+  */
+ #define ece_generic_processing(node) \
+     expression_tree_mutator((Node *) (node), eval_const_expressions_mutator, \
+                             (void *) context)
+
+ /*
+  * Check whether all arguments of the given node were reduced to Consts.
+  * By going directly to expression_tree_walker, contain_non_const_walker
+  * is not applied to the node itself, only to its children.
+  */
+ #define ece_all_arguments_const(node) \
+     (!expression_tree_walker((Node *) (node), contain_non_const_walker, NULL))
+
+ /* Generic macro for applying evaluate_expr */
+ #define ece_evaluate_expr(node) \
+     ((Node *) evaluate_expr((Expr *) (node), \
+                             exprType((Node *) (node)), \
+                             exprTypmod((Node *) (node)), \
+                             exprCollation((Node *) (node))))
+
+ /*
+  * Recursive guts of eval_const_expressions/estimate_expression_value
+  */
  static Node *
  eval_const_expressions_mutator(Node *node,
                                 eval_const_expressions_context *context)
*************** eval_const_expressions_mutator(Node *nod
*** 2830,2835 ****
--- 2864,2888 ----
                  newexpr->location = expr->location;
                  return (Node *) newexpr;
              }
+         case T_ScalarArrayOpExpr:
+             {
+                 ScalarArrayOpExpr *saop;
+
+                 /* Copy the node and const-simplify its arguments */
+                 saop = (ScalarArrayOpExpr *) ece_generic_processing(node);
+
+                 /* Make sure we know underlying function */
+                 set_sa_opfuncid(saop);
+
+                 /*
+                  * If all arguments are Consts, and it's a safe function, we
+                  * can fold to a constant
+                  */
+                 if (ece_all_arguments_const(saop) &&
+                     ece_function_is_safe(saop->opfuncid, context))
+                     return ece_evaluate_expr(saop);
+                 return (Node *) saop;
+             }
          case T_BoolExpr:
              {
                  BoolExpr   *expr = (BoolExpr *) node;
*************** eval_const_expressions_mutator(Node *nod
*** 3054,3100 ****
              }
          case T_ArrayCoerceExpr:
              {
!                 ArrayCoerceExpr *expr = (ArrayCoerceExpr *) node;
!                 Expr       *arg;
!                 Expr       *elemexpr;
!                 ArrayCoerceExpr *newexpr;
!
!                 /*
!                  * Reduce constants in the ArrayCoerceExpr's argument and
!                  * per-element expressions, then build a new ArrayCoerceExpr.
!                  */
!                 arg = (Expr *) eval_const_expressions_mutator((Node *) expr->arg,
!                                                               context);
!                 elemexpr = (Expr *) eval_const_expressions_mutator((Node *) expr->elemexpr,
!                                                                    context);

!                 newexpr = makeNode(ArrayCoerceExpr);
!                 newexpr->arg = arg;
!                 newexpr->elemexpr = elemexpr;
!                 newexpr->resulttype = expr->resulttype;
!                 newexpr->resulttypmod = expr->resulttypmod;
!                 newexpr->resultcollid = expr->resultcollid;
!                 newexpr->coerceformat = expr->coerceformat;
!                 newexpr->location = expr->location;

                  /*
!                  * If constant argument and per-element expression is
                   * immutable, we can simplify the whole thing to a constant.
                   * Exception: although contain_mutable_functions considers
                   * CoerceToDomain immutable for historical reasons, let's not
                   * do so here; this ensures coercion to an array-over-domain
                   * does not apply the domain's constraints until runtime.
                   */
!                 if (arg && IsA(arg, Const) &&
!                     elemexpr && !IsA(elemexpr, CoerceToDomain) &&
!                     !contain_mutable_functions((Node *) elemexpr))
!                     return (Node *) evaluate_expr((Expr *) newexpr,
!                                                   newexpr->resulttype,
!                                                   newexpr->resulttypmod,
!                                                   newexpr->resultcollid);
!
!                 /* Else we must return the partially-simplified node */
!                 return (Node *) newexpr;
              }
          case T_CollateExpr:
              {
--- 3107,3130 ----
              }
          case T_ArrayCoerceExpr:
              {
!                 ArrayCoerceExpr *ac;

!                 /* Copy the node and const-simplify its arguments */
!                 ac = (ArrayCoerceExpr *) ece_generic_processing(node);

                  /*
!                  * If constant argument and the per-element expression is
                   * immutable, we can simplify the whole thing to a constant.
                   * Exception: although contain_mutable_functions considers
                   * CoerceToDomain immutable for historical reasons, let's not
                   * do so here; this ensures coercion to an array-over-domain
                   * does not apply the domain's constraints until runtime.
                   */
!                 if (ac->arg && IsA(ac->arg, Const) &&
!                     ac->elemexpr && !IsA(ac->elemexpr, CoerceToDomain) &&
!                     !contain_mutable_functions((Node *) ac->elemexpr))
!                     return ece_evaluate_expr(ac);
!                 return (Node *) ac;
              }
          case T_CollateExpr:
              {
*************** eval_const_expressions_mutator(Node *nod
*** 3286,3326 ****
                  else
                      return copyObject(node);
              }
          case T_ArrayExpr:
              {
!                 ArrayExpr  *arrayexpr = (ArrayExpr *) node;
!                 ArrayExpr  *newarray;
!                 bool        all_const = true;
!                 List       *newelems;
!                 ListCell   *element;
!
!                 newelems = NIL;
!                 foreach(element, arrayexpr->elements)
!                 {
!                     Node       *e;
!
!                     e = eval_const_expressions_mutator((Node *) lfirst(element),
!                                                        context);
!                     if (!IsA(e, Const))
!                         all_const = false;
!                     newelems = lappend(newelems, e);
!                 }
!
!                 newarray = makeNode(ArrayExpr);
!                 newarray->array_typeid = arrayexpr->array_typeid;
!                 newarray->array_collid = arrayexpr->array_collid;
!                 newarray->element_typeid = arrayexpr->element_typeid;
!                 newarray->elements = newelems;
!                 newarray->multidims = arrayexpr->multidims;
!                 newarray->location = arrayexpr->location;
!
!                 if (all_const)
!                     return (Node *) evaluate_expr((Expr *) newarray,
!                                                   newarray->array_typeid,
!                                                   exprTypmod(node),
!                                                   newarray->array_collid);

!                 return (Node *) newarray;
              }
          case T_CoalesceExpr:
              {
--- 3316,3337 ----
                  else
                      return copyObject(node);
              }
+         case T_ArrayRef:
          case T_ArrayExpr:
+         case T_RowExpr:
              {
!                 /*
!                  * Generic handling for node types whose own processing is
!                  * known to be immutable, and for which we need no smarts
!                  * beyond "simplify if all inputs are constants".
!                  */

!                 /* Copy the node and const-simplify its arguments */
!                 node = ece_generic_processing(node);
!                 /* If all arguments are Consts, we can fold to a constant */
!                 if (ece_all_arguments_const(node))
!                     return ece_evaluate_expr(node);
!                 return node;
              }
          case T_CoalesceExpr:
              {
*************** eval_const_expressions_mutator(Node *nod
*** 3397,3403 ****
                   * simple Var.  (This case won't be generated directly by the
                   * parser, because ParseComplexProjection short-circuits it.
                   * But it can arise while simplifying functions.)  Also, we
!                  * can optimize field selection from a RowExpr construct.
                   *
                   * However, replacing a whole-row Var in this way has a
                   * pitfall: if we've already built the rel targetlist for the
--- 3408,3415 ----
                   * simple Var.  (This case won't be generated directly by the
                   * parser, because ParseComplexProjection short-circuits it.
                   * But it can arise while simplifying functions.)  Also, we
!                  * can optimize field selection from a RowExpr construct, or
!                  * of course from a constant.
                   *
                   * However, replacing a whole-row Var in this way has a
                   * pitfall: if we've already built the rel targetlist for the
*************** eval_const_expressions_mutator(Node *nod
*** 3412,3417 ****
--- 3424,3431 ----
                   * We must also check that the declared type of the field is
                   * still the same as when the FieldSelect was created --- this
                   * can change if someone did ALTER COLUMN TYPE on the rowtype.
+                  * If it isn't, we skip the optimization; the case will
+                  * probably fail at runtime, but that's not our problem here.
                   */
                  FieldSelect *fselect = (FieldSelect *) node;
                  FieldSelect *newfselect;
*************** eval_const_expressions_mutator(Node *nod
*** 3462,3467 ****
--- 3476,3492 ----
                  newfselect->resulttype = fselect->resulttype;
                  newfselect->resulttypmod = fselect->resulttypmod;
                  newfselect->resultcollid = fselect->resultcollid;
+                 if (arg && IsA(arg, Const))
+                 {
+                     Const       *con = (Const *) arg;
+
+                     if (rowtype_field_matches(con->consttype,
+                                               newfselect->fieldnum,
+                                               newfselect->resulttype,
+                                               newfselect->resulttypmod,
+                                               newfselect->resultcollid))
+                         return ece_evaluate_expr(newfselect);
+                 }
                  return (Node *) newfselect;
              }
          case T_NullTest:
*************** eval_const_expressions_mutator(Node *nod
*** 3557,3562 ****
--- 3582,3594 ----
              }
          case T_BooleanTest:
              {
+                 /*
+                  * This case could be folded into the generic handling used
+                  * for ArrayRef etc.  But because the simplification logic is
+                  * so trivial, applying evaluate_expr() to perform it would be
+                  * a heavy overhead.  BooleanTest is probably common enough to
+                  * justify keeping this bespoke implementation.
+                  */
                  BooleanTest *btest = (BooleanTest *) node;
                  BooleanTest *newbtest;
                  Node       *arg;
*************** eval_const_expressions_mutator(Node *nod
*** 3630,3643 ****
      }

      /*
!      * For any node type not handled above, we recurse using
!      * expression_tree_mutator, which will copy the node unchanged but try to
!      * simplify its arguments (if any) using this routine. For example: we
!      * cannot eliminate an ArrayRef node, but we might be able to simplify
!      * constant expressions in its subscripts.
       */
!     return expression_tree_mutator(node, eval_const_expressions_mutator,
!                                    (void *) context);
  }

  /*
--- 3662,3718 ----
      }

      /*
!      * For any node type not handled above, copy the node unchanged but
!      * const-simplify its subexpressions.  This is the correct thing for node
!      * types whose behavior might change between planning and execution, such
!      * as CoerceToDomain.  It's also a safe default for new node types not
!      * known to this routine.
       */
!     return ece_generic_processing(node);
! }
!
! /*
!  * Subroutine for eval_const_expressions: check for non-Const nodes.
!  *
!  * We can abort recursion immediately on finding a non-Const node.  This is
!  * critical for performance, else eval_const_expressions_mutator would take
!  * O(N^2) time on non-simplifiable trees.  However, we do need to descend
!  * into List nodes since expression_tree_walker sometimes invokes the walker
!  * function directly on List subtrees.
!  */
! static bool
! contain_non_const_walker(Node *node, void *context)
! {
!     if (node == NULL)
!         return false;
!     if (IsA(node, Const))
!         return false;
!     if (IsA(node, List))
!         return expression_tree_walker(node, contain_non_const_walker, context);
!     /* Otherwise, abort the tree traversal and return true */
!     return true;
! }
!
! /*
!  * Subroutine for eval_const_expressions: check if a function is OK to evaluate
!  */
! static bool
! ece_function_is_safe(Oid funcid, eval_const_expressions_context *context)
! {
!     char        provolatile = func_volatile(funcid);
!
!     /*
!      * Ordinarily we are only allowed to simplify immutable functions. But for
!      * purposes of estimation, we consider it okay to simplify functions that
!      * are merely stable; the risk that the result might change from planning
!      * time to execution time is worth taking in preference to not being able
!      * to estimate the value at all.
!      */
!     if (provolatile == PROVOLATILE_IMMUTABLE)
!         return true;
!     if (context->estimate && provolatile == PROVOLATILE_STABLE)
!         return true;
!     return false;
  }

  /*
diff --git a/src/test/regress/expected/rowtypes.out b/src/test/regress/expected/rowtypes.out
index 43b36f6..a4bac8e 100644
*** a/src/test/regress/expected/rowtypes.out
--- b/src/test/regress/expected/rowtypes.out
*************** ERROR:  cannot compare dissimilar column
*** 307,316 ****
  explain (costs off)
  select * from int8_tbl i8
  where i8 in (row(123,456)::int8_tbl, '(4567890123456789,123)');
!                                                    QUERY PLAN
! -----------------------------------------------------------------------------------------------------------------
   Seq Scan on int8_tbl i8
!    Filter: (i8.* = ANY (ARRAY[ROW('123'::bigint, '456'::bigint)::int8_tbl, '(4567890123456789,123)'::int8_tbl]))
  (2 rows)

  select * from int8_tbl i8
--- 307,316 ----
  explain (costs off)
  select * from int8_tbl i8
  where i8 in (row(123,456)::int8_tbl, '(4567890123456789,123)');
!                                   QUERY PLAN
! -------------------------------------------------------------------------------
   Seq Scan on int8_tbl i8
!    Filter: (i8.* = ANY ('{"(123,456)","(4567890123456789,123)"}'::int8_tbl[]))
  (2 rows)

  select * from int8_tbl i8