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[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Alexander!

On May 04, Alexander Barkov wrote:
> >> @@ -2513,7 +2530,14 @@ void Item_func_nullif::update_used_tables()
> >>     }
> >>     else
> >>     {
> >> -    Item_func::update_used_tables();
> >> +    /*
> >> +      No needs to iterate through args[2] when it's just a copy of args[0].
> >> +      See MDEV-9712 Performance degradation of nested NULLIF
> >> +    */
> >> +    DBUG_ASSERT(arg_count == 3);
> >> +    used_tables_and_const_cache_init();
> >> +    used_tables_and_const_cache_update_and_join(args[0] == args[2] ? 2 : 3,
> >> +                                                args);
> >
> > here, you can unconditionally run
> >
> >    used_tables_and_const_cache_update_and_join(2, args);
> >
> > because even if args[2] differs from args[0], it cannot use more
> > tables, so for the purpose of this method, it's redundant.
> 
> I'm not sure.
> 
> After equal const propagation, update_used_tables() is called again 2 
> times in this script:
> 
> DROP TABLE IF EXISTS t1;
> CREATE TABLE t1 (a BINARY(1));
> INSERT INTO t1 VALUES ('a'),('b');
> SELECT * FROM t1 WHERE  NULLIF(a,'a') IS NULL AND a='a';
> 
> <offtopic>
> Btw, update_used_tables() is called:
> - 3 times before propagate_equal_fields()
> - 2 times after propagate_equal_fields()
> Perhaps we should generally try to reduce the amount of 
> update_used_tables().
> </offtopic>

Perhaps. We're in general walking the tree too much, I think.

> When update_used_tables() is called after propagate_equal_fields(),
> args[0] can already be a constant, while args[2] can still point to the
> original expression involving tables.
> 
> So args[2] can have more tables, it seems.
> 
> Can you clarify please?

Then I was wrong :)
I thought it'll be just a cast or charset convertion on top of args[0]
(or the other way around), and forgot about constant substitution.

Regards,
Sergei
Chief Architect MariaDB
and security@xxxxxxxxxxx