The Incremental Sort had a couple issues, resulting in leaking memory
during rescans, possibly triggering OOM. The code had a couple of
related flaws:
1. During rescans, the sort states were reset but then also set to NULL
(despite the comment saying otherwise). ExecIncrementalSort then
sees NULL and initializes a new sort state, leaking the memory used
by the old one.
2. Initializing the sort state also automatically rebuilt the info about
presorted keys, leaking the already initialized info. presorted_keys
was also unnecessarily reset to NULL.
Patch by James Coleman, based on patches by Laurenz Albe and Tom Lane.
Backpatch to 13, where Incremental Sort was introduced.
Author: James Coleman, Laurenz Albe, Tom Lane
Reported-by: Laurenz Albe, Zu-Ming Jiang
Backpatch-through: 13
Discussion: https://postgr.es/m/b2bd02dff61af15e3526293e2771f874cf2a3be7.camel%40cybertec.at
Discussion: https://postgr.es/m/db03c582-086d-e7cd-d4a1-3bc722f81765%40inf.ethz.ch
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