Convert Non-correlated EXISTS subquery to a LIMIT 1 AND a JOIN
Enable GPORCA to generate better plans for non-correlated exists subquery in the WHERE clause
Consider the following exists subquery, `(select * from bar)`. GPORCA generates an elaborate count based implementation of this subquery. If bar is a fact table, the count is going to be expensive.
```
vraghavan=# explain select * from foo where foo.a = foo.b and exists (select * from bar);
QUERY PLAN
------------------------------------------------------------------------------------------------------------------
Gather Motion 3:1 (slice3; segments: 3) (cost=0.00..1368262.79 rows=400324 width=8)
-> Nested Loop (cost=0.00..1368250.86 rows=133442 width=8)
Join Filter: true
-> Table Scan on foo (cost=0.00..461.91 rows=133442 width=8)
Filter: a = b
-> Materialize (cost=0.00..438.57 rows=1 width=1)
-> Broadcast Motion 1:3 (slice2) (cost=0.00..438.57 rows=3 width=1)
-> Result (cost=0.00..438.57 rows=1 width=1)
Filter: (count((count()))) > 0::bigint
-> Aggregate (cost=0.00..438.57 rows=1 width=8)
-> Gather Motion 3:1 (slice1; segments: 3) (cost=0.00..438.57 rows=1 width=8)
-> Aggregate (cost=0.00..438.57 rows=1 width=8)
-> Table Scan on bar (cost=0.00..437.95 rows=332395 width=1)
Optimizer status: PQO version 2.35.1
(14 rows)
```
Planner on the other hand uses LIMIT as shown in the INIT plan.
```
vraghavan=# explain select * from foo where foo.a = foo.b and exists (select * from bar);
QUERY PLAN
------------------------------------------------------------------------------------------------
Gather Motion 3:1 (slice2; segments: 3) (cost=0.03..13611.14 rows=1001 width=8)
-> Result (cost=0.03..13611.14 rows=334 width=8)
One-Time Filter: $0
InitPlan (slice3)
-> Limit (cost=0.00..0.03 rows=1 width=0)
-> Gather Motion 3:1 (slice1; segments: 3) (cost=0.00..0.03 rows=1 width=0)
-> Limit (cost=0.00..0.01 rows=1 width=0)
-> Seq Scan on bar (cost=0.00..11072.84 rows=332395 width=0)
-> Seq Scan on foo (cost=0.00..13611.11 rows=334 width=8)
Filter: a = b
Settings: optimizer=off
Optimizer status: legacy query optimizer
(12 rows)
```
While GPORCA doesnot support init-plan, we can nevertheless generate a better plan by using LIMIT instead of count. After this PR, GPORCA will generate the following plan with LIMIT clause.
```
vraghavan=# explain select * from foo where foo.a = foo.b and exists (select * from bar);
QUERY PLAN
------------------------------------------------------------------------------------------------------------
Gather Motion 3:1 (slice3; segments: 3) (cost=0.00..1368262.73 rows=400324 width=8)
-> Nested Loop EXISTS Join (cost=0.00..1368250.80 rows=133442 width=8)
Join Filter: true
-> Table Scan on foo (cost=0.00..461.91 rows=133442 width=8)
Filter: a = b
-> Materialize (cost=0.00..438.57 rows=1 width=1)
-> Broadcast Motion 1:3 (slice2) (cost=0.00..438.57 rows=3 width=1)
-> Limit (cost=0.00..438.57 rows=1 width=1)
-> Gather Motion 3:1 (slice1; segments: 3) (cost=0.00..438.57 rows=1 width=1)
-> Limit (cost=0.00..438.57 rows=1 width=1)
-> Table Scan on bar (cost=0.00..437.95 rows=332395 width=1)
Optimizer status: PQO version 2.35.1
(12 rows)
```
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