Re: Explicit deterministic COLLATE fails with pattern matching operations on column with non-deterministic collation

On Wed, May 27, 2020 at 8:23 AM James Lucas <jlucasdba@gmail.com> wrote:

>
> create table ctestnd (id numeric, t text collate mycollation);
>
> create index ctestnd_idx02 on ctestnd (t collate "C");
>


> Test on ctestnd:
> explain select * from ctestnd where t = 'aAa' collate "C";
>                                   QUERY PLAN
>
> ------------------------------------------------------------------------------
>  Index Scan using ctestnd_idx02 on ctestnd  (cost=0.42..4.44 rows=1
> width=10)
>    Index Cond: (t = 'aAa'::text COLLATE "C")
> COMMENT: Works as expected.
>

Uses an index scan which is where the deterministic collation exists


>
> explain select * from ctestnd where t like 'a%';
> ERROR:  nondeterministic collations are not supported for LIKE
> COMMENT: Fails as expected.
>
> explain select * from ctestnd where t like 'a%' collate "C";
> ERROR:  nondeterministic collations are not supported for LIKE
>
>
Your schema is inherently unstable in this respect because the planner has
to be allowed to choose a sequential scan and as soon as it does it
attempts to perform like comparisons with table data that is stored using a
non-deterministic collation.

I don't know what kinds of promises we make about implicit collation
manipulation here but absent such a transformation the sequential scan plan
with LIKE generates an invalid plan choice.  That it doesn't go find the
index that happens to have a workable collation for the query is
unsurprising - whether that is even a possibility is beyond me.

David J.