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MWL#182: a problem select_type column, SIMPLE vs PRIMARY



>>>>> "Sergey" == Sergey Petrunya <psergey@xxxxxxxxxxxx> writes:

Sergey> Hello,
Sergey> When coding MWL#182, I've found out that it is very difficult to print
Sergey> the same value of select_type as EXPLAIN does. 


Sergey> Future solution #1: get rid of SIMPLE
Sergey> -------------------------------------

Sergey> I don't see a value of having select_type=SIMPLE (other than term "simple"
Sergey> being encouraging for novice users:)  We could change SIMPLE to PRIMARY
Sergey> everywhere and get rid of the problem

Sergey> Future solution #2: use SIMPLE/PRIMARY approach of SHOW EXPLAIN
Sergey> ---------------------------------------------------------------
Sergey> EXPLAIN SELECT produces incorrect values because it calls 
Sergey> st_select_lex::set_explain_type() before the query rewrites are done, so 
Sergey> it can't see that
Sergey> - used VIEWs have subqueries
Sergey> - used subqueries will be flattened
Sergey> - etc.

Sergey> SHOW EXPLAIN operates on a query that is being executed, and that alone
Sergey> guarantees its output is closer to reality. This means, EXPLAIN SELECT output
Sergey> must be changed to match SHOW EXPLAIN output.  This is a lot of changes 
Sergey> across a number of .result files.

It don't think it's a big problem if 'show explain' have a slightly
different result than 'explain select' for the time being (if it's more