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BUG #18652: Planner can not find pathkey item to sort for query with expression and expression index

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From: PG Bug reporting form <noreply@postgresql.org>
To: pgsql-bugs@lists.postgresql.org
Cc: exclusion@gmail.com
Date: 2024-10-09T21:00:01Z
Lists: pgsql-bugs 
The following bug has been logged on the website:

Bug reference:      18652
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 17.0
Operating system:   Ubuntu 22.04
Description:        

The following query:
CREATE TABLE t(i int, j int);
CREATE INDEX idx on t((i + 0));

SELECT * FROM t,
  (SELECT i + 0 AS i FROM
    (SELECT i FROM t UNION ALL SELECT i + 1 FROM t) AS t1
  ) AS t2
WHERE t2.i = t.j;

fails with:
ERROR:  XX000: could not find pathkey item to sort
LOCATION:  prepare_sort_from_pathkeys, createplan.c:6350

The error occurs only when the expression in SELECT i + 0 AS i FROM
matches the expression in an index.

Reproduced on REL_10_STABLE .. master.

Commits

  1. Correctly identify which EC members are computable at a plan node.

  2. Support MergeAppend plans, to allow sorted output from append relations.