Fix DISTINCT queries with LIMIT and no ORDER on SQLServer 2012

[billschaller]

On SQLServer2012Platform, doModifyLimitQuery adds a do-nothing ORDER BY clause, because this is required in order to use OFFSET...FETCH NEXT N ROWS ONLY.

DISTINCT queries run via the paginator without an ORDER BY clause on SQL Server 2012 were failing with this error:

ORDER BY items must appear in the select list if SELECT DISTINCT is specified.

This PR fixes the error by adding 0 to the select list and changing the generated ORDER BY clause to read ORDER BY 1.

So a query that would have been generated as:

SELECT DISTINCT id_0
FROM (
    SELECT p0_.id AS id_0
        ,p0_.preference_code AS preference_code_1
        ,p0_.id_zone AS id_zone_2
    FROM preference p0_
    WHERE (p0_.id_zone IN (2))
    ) dctrn_result
ORDER BY (SELECT 0) 
OFFSET 0 ROWS FETCH NEXT 30 ROWS ONLY

Will now be generated as:

SELECT DISTINCT 0, id_0
FROM (
    SELECT p0_.id AS id_0
        ,p0_.preference_code AS preference_code_1
        ,p0_.id_zone AS id_zone_2
    FROM preference p0_
    WHERE (p0_.id_zone IN (2))
    ) dctrn_result
ORDER BY 1
OFFSET 0 ROWS FETCH NEXT 30 ROWS ONLY

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[deeky666]

Nitpicking: stripos($query, 'SELECT DISTINCT') === 0

[Ocramius]

Since it's quite the madness: is there a way to integration-test this as well?

[billschaller]

There is, but there is no existing functional test suite for DBAL that covers this kind of stuff, and I don't have time to write one. Ideally we should have a testsuite that covers actually executing these queries on all platforms, but we don't :(

I've been using the ORM as my integration testsuite for DBAL, bad as that is.

[deeky666]

I'm fine with this PR as is for now. We should add the integration tests for the limit/offset modifications in seperate PR anyways.

[deeky666]

Thanks @zeroedin-bill