Fix bug around assignment expressions containing indirections.

Andres Freund <andres@anarazel.de>

Commit: bb0203f26fa5f09fe2689a9db4bc632c1435edec
Author: Andres Freund <andres@anarazel.de>
Date: 2015-07-24T09:52:22Z
Releases: 9.5.0
Fix bug around assignment expressions containing indirections.

Handling of assigned-to expressions with indirection (e.g. set f1[1] =
3) was broken for ON CONFLICT DO UPDATE.  The problem was that
ParseState was consulted to determine if an INSERT-appropriate or
UPDATE-appropriate behavior should be used when transforming expressions
with indirections. When the wrong path was taken the old row was
substituted with NULL, leading to wrong results..

To fix remove p_is_update and only use p_is_insert to decide how to
transform the assignment expression, and uset p_is_insert while parsing
the on conflict statement. This isn't particularly pretty, but it's not
any worse than before.

Author: Peter Geoghegan, slightly edited by me
Discussion: CAM3SWZS8RPvA=KFxADZWw3wAHnnbxMxDzkEC6fNaFc7zSm411w@mail.gmail.com
Backpatch: 9.5, where the feature was introduced

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