Don't execute sql query with limit 0

[Alexander Schepanovski]

When QuerySet.query has low_mark == high_mark (happens in pagination for example) ORM executes a query like:

    select ... from ... limit 0

[Ramiro Morales]

Don't query the DB when we know it's going to have LIMIT 0

[Ramiro Morales]

This one doesn even generate the SQL of the query if it's going to have LIMIT 0

[Ramiro Morales]

I've attached two patches that implement detection of the high_mark == low_mark condition and avoid going to the DB with such a query, the second patch could be even slightly more efficient because it shortcuts execution of Query.as_sql() early completely avoiding the setup and generation of the SQL query text.

One thing to review and decide is that isn't clear from the docstrings and comments what Query.as_sql() is supposed to return to signal no SQL query text was generated. The patches currently return an empty string but I don't know now if it should be None.

Tests that exercise this already exist in modeltests/basic and regressiontests/queries.

[Ramiro Morales]

I've updated the patches for this ticket to trunk status after the multi-db merge.

If/once one of the are chosen and applied to trunk, then the similarly named chosen among 12192-patch1.diff and 12192-patch2.diff could be applied to 1.1.x as they should still apply correctly there.

[Ramiro Morales]

Patches were fixed and updated to r12492. Both of them were tested by running the Django test suite against MySQL, PostgreSQL and sqlite3 and it was verified none of them introduced errors.

[Ramiro Morales]

Patch for review

[Ramiro Morales]

(In [14204]) Fixed #12192 -- Don't execute any DB query when the QS slicing being performed
will result in use of LIMIT 0. Thanks Suor for the report.