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Re: problem with partitioning and our storage engine in 5.2



I think we found out the problem. In ha_partition.cc, in the function
ha_partition::handle_ordered_index_scan, this code was added:
     case partition_index_last:
         MyISAM engine can fail if we call index_last() when indexes disabled
         that happens if the table is empty.
         Here we use file->stats.records instead of file->records() because
         file->records() is supposed to return an EXACT count, and it can be
         possibly slow. We don't need an exact number, an approximate one- from
         the last ::info() call - is sufficient.
       if (file->stats.records == 0)
         error= HA_ERR_END_OF_FILE;

We always thought that stats.records was meant to be an estimate, and
that an estimate of 0 was ok even if the table was non-empty. We were
reporting that stats.records was 0 even though the table was
non-empty. Is this assumption wrong?

The assumption is not generally wrong, however there are two kinds of
storage engines in this respect. The first kind promises to provide
exact count of rows, MyISAM is an example. The second kind doesn't
promise exact counts, for instance InnoDB. The optimizer checks this,
and if it can rely on the exact count, then it optimizes count(*)
whenever possible (for count(*) queries without GROUP BY).

The relevant code is in opt_sum.cc. The function opt_sum_query
performs this test:
    if (!(tl->table->file->ha_table_flags() & HA_STATS_RECORDS_IS_EXACT) ||

The question is what is your engine's implementation of
your_handler_class::ha_table_flags()? If you "lie" to the optimizer
that the count is exact, but then treat is an approximate value,
you'll get the above wrong result for sure.

If this is not the problem, then please post a reproducible example.



On Wed, Dec 8, 2010 at 2:42 PM, Kristian Nielsen
<knielsen@xxxxxxxxxxxxxxx>  wrote:
Zardosht Kasheff<zardosht@xxxxxxxxx>  writes:

We have been working on testing our storage engine, TokuDB, against
MariaDB 5.2.3, and we have encountered a problem with partitioning
that does not exist on MySQL 5.1, MySQL 5.5, and MariaDB 5.1.50. This
problem also does not exist with any other storage engine that we have
tried. It ONLY exists with TokuDB and MariaDB 5.2.

Here is what the test is doing:

    1. Create a table with some partitions
    2. update the table
    3. run select max(f_int1)
    4. run select *
    5. run select max(f_int1) again.

The problem is that the query results for 3 are incorrect, even though
the query results for 4 and 5 are correct. MySQL 5.1, MySQL 5.5, and
MariaDB 5.1 produce the correct results for 3.

What makes this even stranger is that the query results for 4 and 5
are always correct.

It's hard to say without any code. Can you post link to launchpad tree or
source tarball with which to reproduce?

 From the information given, I would suggest to run the test with --valgrind,
in case the problem is some uninitialised memory being referenced; sometimes
Valgrind can help catch this.

  - Kristian.

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