The idea is that instead of marking all select_lex's with DISTINCT, we
only mark those that really need distinct result.
Benefits of this change:
- Temporary tables used with derived tables, UNION, IN are now smaller
as duplicates are removed already on the insert phase.
- The optimizer can now produce better plans with EQ_REF. This can be
seen from the tests where several queries does not anymore materialize
derived tables twice.
- Queries affected by 'in_predicate_conversion_threshold' where large IN
lists are converted to sub query produces better plans.
Other things:
- Removed on duplicate call to sel->init_select() in
LEX::add_primary_to_query_expression_body()
- I moved the testing of
tab->table->pos_in_table_list->is_materialized_derived()
in join_read_const_table() to the caller as it caused problems for
derived tables that could be proven to be const tables.
This also is likely to fix some bugs as if join_read_const_table()
was aborted, the table was left marked as JT_CONST, which cannot
be good. I added an ASSERT there for now that can be removed when
the code has been properly tested.
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