From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
To: Jeevan Chalke <jeevan.chalke@enterprisedb.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Soon-to-be-broken regression test case
Date: Thu, 11 Oct 2018 12:34:28 -0400
Message-ID: <5395.1539275668@sss.pgh.pa.us> (raw)
The last test case in select_parallel.sql, added in commit dc1057fc,
currently generates a plan like this:
CREATE VIEW tenk1_vw_sec WITH (security_barrier) AS SELECT * FROM tenk1;
EXPLAIN (COSTS OFF)
SELECT 1 FROM tenk1_vw_sec WHERE EXISTS (SELECT 1 WHERE unique1 = 0);
QUERY PLAN
-------------------------------------------------------------------
Subquery Scan on tenk1_vw_sec
Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)
-> Gather
Workers Planned: 4
-> Parallel Index Only Scan using tenk1_unique1 on tenk1
SubPlan 1
-> Result
One-Time Filter: (tenk1_vw_sec.unique1 = 0)
SubPlan 2
-> Result
(10 rows)
I have been fooling around with a patch to allow pull-up of sub-selects
that lack any FROM, along the lines discussed in
https://www.postgresql.org/message-id/15944.1521127664@sss.pgh.pa.us
I find that it is smart enough to reduce that EXISTS to a plain
expression, yielding
QUERY PLAN
----------------------------------------------------
Subquery Scan on tenk1_vw_sec
-> Index Only Scan using tenk1_unique1 on tenk1
Index Cond: (unique1 = 0)
(3 rows)
While that's obviously a far better plan, it does not meet this test
case's stated goal of testing the interaction of subqueries with
parallel query. Could you suggest a less trivial subquery that will
still do what you intended?
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, jeevan.chalke@enterprisedb.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Soon-to-be-broken regression test case
In-Reply-To: <5395.1539275668@sss.pgh.pa.us>
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