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Re: Likely a dead optimizer related code



On 07/09/2015 05:00 PM, Michael Widenius wrote:

Hi Igor,

I noticed that during Item_cond::fix_fields() and
Item_func_between::fix_fields() update these optimizer
related SELECT_LEX members:

thd->lex->current_select->cond_count and

The purpose of these members is to allocate optimizer related buffers
- update_ref_and_keys() in sql_select.cc and
- check_func_dependency() in opt_table_elimination,cc
and it seems they have no any other purposes..


  From my understanding, this is a dead code and the collected values are
never used:

- cond_count and_between_count are later always initialized to 0
before the walk(count_sargable_conds) call in update_ref_and_keys().

And the call to check_func_dependency() is done after this call.

- moreover, using the values collected during fix_fields() would not be
correct to allocate optimized related buffers, because the code
in fix_fields() does not distinguish between different query parts,
so the items can be just a part of the SELECT list rather than a

This would note be a problem as the count was supposed to be an upper bound.
As it's not normal that you have many IF or BETWEEN in the SELECT
part, this was not a problem.

I looked at the patch and it's ok to commit.

Thanks. I created a bug report, for reference:

MDEV-8456 Dead code in Item_cond::fix_fields() and Item_func_between::fix_fields()

and pushed into 10.1.