pgsql: Test what BEFORE UPDATE triggers do to FOR PORTION OF

Test what BEFORE UPDATE triggers do to FOR PORTION OF

If a BEFORE trigger changes NEW.valid_at, what is the interaction with
FOR PORTION OF? This commit gives a test to capture our current
behavior: The trigger's change replaces the value we computed
automatically, but it does not change the bounds of the temporal
leftovers.

This matches the behavior of MariaDB. On the other hand, DB2 rejects
changing the start/end columns of a PERIOD. Since we don't have
PERIODs, we can't reject the change at trigger definition time as DB2
does, but we could reject it at run time by comparing the values
before and after running triggers.

Author: Paul A. Jungwirth <pj(at)illuminatedcomputing(dot)com>
Discussion: https://www.postgresql.org/message-id/CA%2BrenyV3Cr9BvWsPeb1t8b%3DPk24apuzyGbubAEs_YsgLUTfXpg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9170c8b71693cf76a4b9a9fbcd16b1fb20b3357c

Modified Files
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src/test/regress/expected/for_portion_of.out | 47 ++++++++++++++++++++++++++++
src/test/regress/sql/for_portion_of.sql | 47 ++++++++++++++++++++++++++++
2 files changed, 94 insertions(+)