From: Jorgen Loland
Date: January 14 2011 12:22pm
Subject: Re: bzr commit into mysql-next-mr-bugfixing branch (jorgen.loland:3248)
 WL#5741
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On 01/11/2011 02:44 PM, Guilhem Bichot wrote:
> Hello again,
>
> This is fine, I just have curiosity questions:
>
> Jorgen Loland a écrit, Le 11.01.2011 12:56:
>> + "records_estimation": [
>> + {
>> + "database": "test",
>> + "table": "t2",
>> + "records": 1,
>
> Do you know why it says there is 1 record, though the table is empty?

The code doesn't check how many rows there are, but the fact that it has type 
system or const:

         if (s->type == JT_SYSTEM || s->type == JT_CONST)
         {
           trace_table.add("records", 1).add("cost", 1);
           trace_table.add_alnum("table_type", (s->type == JT_SYSTEM) ?
                                 "system": "const");

           /* Only one matching row */
           s->found_records= s->records= s->read_time=1; s->worst_seeks= 1.0;
           continue;
         }


>> + ] /* evaluate_subselect_cond_steps */,
>> + "after_equality_propagation": "((NULL <> (/* select#3 */ select
>> `test`.`t2`.`a` from `test`.`t2` limit 1)) and multiple equal((/*
>> select#2 */ select NULL from `test`.`t2` limit 1), `test`.`t1`.`a`))",
>
> I don't understand how we come to this new condition above, from
> "original_condition":
> * "NULL <> (select #3 etc)"
> makes me think that t1.a was replaced by NULL (but why: t1 is empty?).
> * in multiple equal((/* select#2 */ select NULL from `test`.`t2` limit
> 1), `test`.`t1`.`a`) t1.a was not replaced, and also t2.b was replaced
> by NULL (which may be ok if the system noted that t2 is empty and thus
> the subquery will return NULL, but then why didn't it replace the entire
> subquery with NULL?).
> This is not related to your patch, so if you don't know, I can research
> it myself.

I have no idea.

-- 
Jørgen Løland | Senior Software Engineer | +47 73842138
Oracle MySQL
Trondheim, Norway