Re: what can a sequence of handler::index_XXX be interrupted by?

On Wed, Jul 27, 2011 at 10:44:22PM -0400, Zardosht Kasheff wrote:
> Actually, let me put the question another way.
> 
> Given that we have had one call of handler::index_init with some thd
> and keynr, are we guaranteed to NOT get another call to
> handler::index_init with the SAME thd and keynr, until we get a call
> to handler::index_end for the first call?

No. It's easiest to show with a counterexample:

create table parent (a int, b int);
insert into parent values (1,1),(2,2); 
create table child1(a int, key(a));
create table child2(b int, key(b));
insert into child1 select a from parent;
insert into child1 select b from parent;

MariaDB [test]> explain select * from parent P, child1 C1, child2 C2 where P.a=C1.a and P.b=C2.b;
+----+-------------+-------+------+---------------+------+---------+----------+------+--------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref      | rows | Extra                    |
+----+-------------+-------+------+---------------+------+---------+----------+------+--------------------------+
|  1 | SIMPLE      | P     | ALL  | NULL          | NULL | NULL    | NULL     | 2 |                          |
|  1 | SIMPLE      | C1    | ref  | a             | a    | 5       | test.P.a | 1 | Using where; Using index |
|  1 | SIMPLE      | C2    | ref  | b             | b    | 5       | test.P.b | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+------+---------+----------+------+--------------------------+
3 rows in set (0.01 sec)

(gdb) b ha_myisam::index_init
  Breakpoint 1 at 0x83eb12e: file ../../../sql/handler.h, line 2013.
(gdb) b ha_myisam::index_end
  Breakpoint 2 at 0x83eb139: file ../../../sql/handler.h, line 2014.

MariaDB [test]> select * from parent P, child1 C1, child2 C2 where P.a=C1.a and P.b=C2.b;

  Breakpoint 1, handler::index_init (this=0xafdd9f0, idx=0, sorted=false) at ../../../sql/handler.h:2013
(gdb) c
  Continuing.

  Breakpoint 1, handler::index_init (this=0xafe1568, idx=0, sorted=false) at ../../../sql/handler.h:2013
(gdb) c
  Continuing.
  
  Breakpoint 2, handler::index_end (this=0xafdd9f0) at ../../../sql/handler.h:2014
(gdb) c
  Continuing.
  
  Breakpoint 2, handler::index_end (this=0xafe1568) at ../../../sql/handler.h:2014
(gdb) c
  Continuing.

Breakpoint hits with this=0xafdd9f0 are for table C1, and those with this=0xafe1568 are for table C2

BR
 Sergey
-- 
Sergey Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog

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