Re: PostgreSQL 10 changes in exclusion constraints - did something change? CASE WHEN behavior oddity

From: "Regina Obe" <lr(at)pcorp(dot)us>
To: "'PostgreSQL-development'" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PostgreSQL 10 changes in exclusion constraints - did something change? CASE WHEN behavior oddity
Date: 2017-05-26 05:13:43
Message-ID: 000001d2d5de$d8d66170$8a832450$@pcorp.us
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> Did something change with how exclusion constraints are handled? I'm
trying to troubleshoot a regression we are having with PostGIS raster
support.

> As best I can guess, it's because exclusion constraints that used to work
in past versions are failing in PostgreSQL 10 with an error something like
> this:

> ERROR: conflicting key value violates exclusion constraint
"enforce_spatially_unique_test_raster_columns_rast"
> ERROR: new row for relation "test_raster_columns" violates check
constraint "enforce_coverage_tile_rast"

> Unfortunately I don't know how long this has been an issue since we had an
earlier test failing preventing the raster ones from being tested.

> Thanks,
> Regina

I figured out the culprit was the change in CASE WHEN behavior with set
returning functions

Had a criteria something of the form:

CASE WHEN some_condition_dependent_on_sometable_that_resolves_to_false THEN
(regexp_matches(...))[1] ELSE ... END
FROM sometable;

One thing that seems a little odd to me is why these return a record

SELECT CASE WHEN strpos('ABC', 'd') > 1 THEN (regexp_matches('a (b) c',
'd'))[1] ELSE 'a' END;

SELECT CASE WHEN false THEN (regexp_matches('a (b) c', 'd'))[1] ELSE 'a' END
FROM pg_tables;

And this doesn't - I'm guessing it has to do with this being a function of
the value of table, but it seems unintuitive
From a user perspective.

SELECT CASE WHEN strpos(f.tablename, 'ANY (ARRAY[') > 1 THEN
(regexp_matches('a (b) c', 'd'))[1] ELSE 'a' END
FROM pg_tables AS f;

Pre-PostgreSQL 10 this would return a row for each record in pg_tables

Thanks,
Regina

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