src/backend/optimizer/README.orca

NOT_EXIST_SUBLINK Subplan used inside GPORCA
============================================

GPORCA uses NOT_EXIST_SUBLINK to implement co-related left anti semijoin.
Therefore we still need this in the executor for GPORCA plans.

Example:

```sql
CREATE TABLE csq_r(a int) distributed by (a);
INSERT INTO csq_r VALUES (1);
CREATE OR REPLACE FUNCTION csq_f(a int) RETURNS int AS $$ select $1 $$ LANGUAGE SQL CONTAINS SQL;
explain SELECT * FROM csq_r WHERE not exists (SELECT * FROM csq_f(csq_r.a));
Physical plan:
+--CPhysicalMotionGather(master)   rows:1   width:34  rebinds:1   cost:882688.037301   origin: [Grp:7, GrpExpr:14]
   +--CPhysicalCorrelatedLeftAntiSemiNLJoin("" (8))   rows:1   width:34  rebinds:1   cost:882688.037287   origin: [Grp:7, GrpExpr:13]
      |--CPhysicalTableScan "csq_r" ("csq_r")   rows:1   width:34  rebinds:1   cost:431.000019   origin: [Grp:0, GrpExpr:1]
      |--CPhysicalComputeScalar   rows:1   width:1  rebinds:1   cost:0.000002   origin: [Grp:5, GrpExpr:1]
      |  |--CPhysicalConstTableGet Columns: ["" (8)] Values: [(1)]   rows:1   width:1  rebinds:1   cost:0.000001   origin: [Grp:1, GrpExpr:1]
      |  +--CScalarProjectList   origin: [Grp:4, GrpExpr:0]
      |     +--CScalarProjectElement "csq_f" (9)   origin: [Grp:3, GrpExpr:0]
      |        +--CScalarIdent "a" (0)   origin: [Grp:2, GrpExpr:0]
      +--CScalarConst (1)   origin: [Grp:8, GrpExpr:0]
",
                                   QUERY PLAN
---------------------------------------------------------------------------------
 Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..882688.04 rows=1 width=4)
   Output: a
   ->  Table Scan on public.csq_r  (cost=0.00..882688.04 rows=1 width=4)
         Output: a
         Filter: (SubPlan 1)
         SubPlan 1  (slice1; segments: 3)
           ->  Result  (cost=0.00..0.00 rows=1 width=1)
                 Output:
                 ->  Result  (cost=0.00..0.00 rows=1 width=1)
                       Output: $0,
                       ->  Result  (cost=0.00..0.00 rows=1 width=1)
                             Output: true
 Optimizer: Pivotal Optimizer (GPORCA) version 2.55.2
(13 rows)
```