mardi 29 novembre 2011

Understanding the unique_subquery optimization

If you use the EXPLAIN SELECT statement to see how your subqueries are treated by MySQL, you may sometimes meet the "unique_subquery" optimization. Here is how the manual describes it:
"unique_subquery: this type replaces ref for some IN subqueries of the following form: value IN (SELECT primary_key FROM single_table WHERE some_expr); unique_subquery is just an index lookup function that replaces the subquery completely for better efficiency".
Few weeks ago, while I was reviewing a patch fixing a bug in unique_subquery, I got a "simplification" pulsion. I told myself that:
  •  unique_subquery is an optimization for a special case of simple subqueries (single inner table, using index, no aggregates);
  • we have a more general system around, used for more complex subqueries, naturally capable of handling simple ones too if we wanted;
  • this general system does not have the bug in question...
Then I wondered: what if we removed the unique_subquery optimization, and let the general system handle this simple subquery? This would certainly simplify code, and thus maintainance...But before removing it, of course, we should check whether unique_subquery brings a significant performance benefit.

So today I'm testing unique_subquery against the DBT3 benchmark. I grab a copy of MySQL 5.6.3, and focus on the sixteenth query of DBT3, which contains a subquery (in red) suitable for handling by unique_subquery:


select
 p_brand,
 p_type,
 p_size,
 count(distinct ps_suppkey) as supplier_cnt
from
 partsupp,
 part
where
 p_partkey = ps_partkey
 and p_brand <> 'Brand#23'
 and p_type not like 'LARGE PLATED%'
 and p_size in (43, 1, 25, 5, 35, 12, 42, 40)
 and ps_suppkey not in (
  select
   s_suppkey
  from
   supplier
  where
   s_comment like '%Customer%Complaints%'
 )
group by
 p_brand,
 p_type,
 p_size
order by
 supplier_cnt desc,
 p_brand,
 p_type,
 p_size;

This query executes in 0.65 seconds on my Linux box, and EXPLAIN is:

+----+--------------------+----------+-----------------+----------------------+--------------+---------+---------------------+--------+----------------------------------------------+
| id | select_type        | table    | type            | possible_keys        | key          | key_len | ref                 | rows   | Extra                                        |
+----+--------------------+----------+-----------------+----------------------+--------------+---------+---------------------+--------+----------------------------------------------+
|  1 | PRIMARY            | part     | ALL             | PRIMARY              | NULL         | NULL    | NULL                | 199498 | Using where; Using temporary; Using filesort |
|  1 | PRIMARY            | partsupp | ref             | PRIMARY,i_ps_partkey | i_ps_partkey | 4       | dbt3.part.p_partkey |      2 | Using where; Using index                     |
|  2 | DEPENDENT SUBQUERY | supplier | unique_subquery | PRIMARY              | PRIMARY      | 4       | func                |      1 | Using where                                  |
+----+--------------------+----------+-----------------+----------------------+--------------+---------+---------------------+--------+----------------------------------------------+

When I disable unique_subquery (by modifying MySQL's C++ code), EXPLAIN becomes:

+----+--------------------+----------+--------+----------------------+--------------+---------+---------------------+--------+----------------------------------------------+
| id | select_type        | table    | type   | possible_keys        | key          | key_len | ref                 | rows   | Extra                                        |
+----+--------------------+----------+--------+----------------------+--------------+---------+---------------------+--------+----------------------------------------------+
|  1 | PRIMARY            | part     | ALL    | PRIMARY              | NULL         | NULL    | NULL                | 199498 | Using where; Using temporary; Using filesort |
|  1 | PRIMARY            | partsupp | ref    | PRIMARY,i_ps_partkey | i_ps_partkey | 4       | dbt3.part.p_partkey |      2 | Using where; Using index                     |
|  2 | DEPENDENT SUBQUERY | supplier | eq_ref | PRIMARY              | PRIMARY      | 4       | func                |      1 | Using where                                  |
+----+--------------------+----------+--------+----------------------+--------------+---------+---------------------+--------+----------------------------------------------+

The only change, as expected, is that "unique_subquery" becomes "eq_ref". The used index is the same (the primary key of the "supplier" table). The optimizer has the same notion of unicity: "unique_subquery" and "eq_ref" both denote that a single lookup is needed, as the index is UNIQUE. Same index, same number of lookups: execution could well be as fast with "eq_ref" as it was with "unique_subquery".
But... no. Query now executes in 0.80 seconds. 23% slower than with unique_subquery!

Finer-grained timing shows that the extra 0.15 seconds are indeed lost in the subquery evaluation code.

To understand this, let's follow the execution in detail, based on EXPLAIN output above.
  • First line of EXPLAIN output: we do a table scan on the "part" table ("type=ALL" means "table scan") . The "rows" column of EXPLAIN suggests that we are going to have 199,498 rows of "part".
  • Second line of EXPLAIN output: for each row from the "part" table, we do an index lookup ("ref") into the "i_ps_partkey" index of the "partsupp" table; apparently such lookup will find two rows ("rows=2").
  • At this point, we have a row made of needed columns of "part" and of "partsupp". An upper estimate of the number of those rows is 199,498 multiplied by 2: 400,000. Actually, the real number is around 120,000 (there has been filtering going on, as the "Using where" indicates).
  • Then we evaluate the WHERE clause and thus the "NOT IN (subquery)" predicate (the "DEPENDENT SUBQUERY"). 120,000 evaluations of such predicate. And that's where the difference is.
EXPLAIN EXTENDED and then SHOW WARNINGS show how the predicate
looks like. Let's start with the case where unique_subquery is disabled:


/* select#1 */ select `dbt3`.`part`.`p_brand` AS `p_brand`,`dbt3`.`part`.`p_type` AS `p_type`,`dbt3`.`part`.`p_size` AS `p_size`,count(distinct `dbt3`.`partsupp`.`ps_suppkey`) AS `supplier_cnt` from `dbt3`.`partsupp` join `dbt3`.`part` where ((`dbt3`.`partsupp`.`ps_partkey` = `dbt3`.`part`.`p_partkey`) and (`dbt3`.`part`.`p_brand` <> 'Brand#23') and (not((`dbt3`.`part`.`p_type` like 'LARGE PLATED%'))) and (`dbt3`.`part`.`p_size` in (43,1,25,5,35,12,42,40)) and (not(<in_optimizer>(`dbt3`.`partsupp`.`ps_suppkey`,<exists>(/* select#2 */ select 1 from `dbt3`.`supplier` where ((`dbt3`.`supplier`.`s_comment` like '%Customer%Complaints%') and (<cache>(`dbt3`.`partsupp`.`ps_suppkey`) = `dbt3`.`supplier`.`s_suppkey`))))))) group by `dbt3`.`part`.`p_brand`,`dbt3`.`part`.`p_type`,`dbt3`.`part`.`p_size` order by count(distinct `dbt3`.`partsupp`.`ps_suppkey`) desc,`dbt3`.`part`.`p_brand`,`dbt3`.`part`.`p_type`,`dbt3`.`part`.`p_size`

Above, the part in red says that

ps_suppkey not in (
        select
            s_suppkey
        from
            supplier
        where
            s_comment like '%Customer%Complaints%'
    )
has been transformed from "IN(non correlated subquery)" to "EXISTS(correlated subquery)", yielding this:


not exists (
        select
            1
        from
            supplier
        where
            s_comment like '%Customer%Complaints%'
                        AND s_suppkey = ps_suppkey
        )

or, more exactly (leaving out the NOT operator, for brevity):


<exists>(/* select#2 */ select 1 from `dbt3`.`supplier`
           where ((`dbt3`.`supplier`.`s_comment` like '%Customer%Complaints%')
           and (<cache>(`dbt3`.`partsupp`.`ps_suppkey`) = `dbt3`.`supplier`.`s_suppkey`)))

Evaluating this EXISTS() evaluates the new subquery. This means all the subquery evaluation machinery: calls to JOIN::exec(), sub_select(), evaluate_join_record()... Sure, deep down it does an index lookup like unique_subquery does, but all those function calls have a cost, and so has all the logic which is lying around ready to handle any complexity in the subquery, as this is generic subquery evaluation code ("if group_by do this", "if order_by do this", "if left_join do this": none of those if()s are entered, but deciding to enter them or not has a cost). Plus some initialization code. Plus some de-initialization code. This overhead, repeated 120,000 times, amounts to 0.15 seconds...

Now, EXPLAIN EXTENDED when unique_subquery is enabled:


/* select#1 */ select `dbt3`.`part`.`p_brand` AS `p_brand`,`dbt3`.`part`.`p_type` AS `p_type`,`dbt3`.`part`.`p_size` AS `p_size`,count(distinct `dbt3`.`partsupp`.`ps_suppkey`) AS `supplier_cnt` from `dbt3`.`partsupp` join `dbt3`.`part` where ((`dbt3`.`partsupp`.`ps_partkey` = `dbt3`.`part`.`p_partkey`) and (`dbt3`.`part`.`p_brand` <> 'Brand#23') and (not((`dbt3`.`part`.`p_type` like 'LARGE PLATED%'))) and (`dbt3`.`part`.`p_size` in (43,1,25,5,35,12,42,40)) and (not(<in_optimizer>(`dbt3`.`partsupp`.`ps_suppkey`,<exists>(<primary_index_lookup>(<cache>(`dbt3`.`partsupp`.`ps_suppkey`) in supplier on PRIMARY where ((`dbt3`.`supplier`.`s_comment` like '%Customer%Complaints%') and (<cache>(`dbt3`.`partsupp`.`ps_suppkey`) = `dbt3`.`supplier`.`s_suppkey`)))))))) group by `dbt3`.`part`.`p_brand`,`dbt3`.`part`.`p_type`,`dbt3`.`part`.`p_size` order by count(distinct `dbt3`.`partsupp`.`ps_suppkey`) desc,`dbt3`.`part`.`p_brand`,`dbt3`.`part`.`p_type`,`dbt3`.`part`.`p_size`

The optimizer has first done the same transformation (IN to EXISTS) as we saw before, then has done one more transformation, and EXISTS has become, as written in red above:


<exists>(<primary_index_lookup>(<cache>(`dbt3`.`partsupp`.`ps_suppkey`)
          in supplier on PRIMARY
          where ((`dbt3`.`supplier`.`s_comment` like '%Customer%Complaints%')
          and (<cache>(`dbt3`.`partsupp`.`ps_suppkey`) = `dbt3`.`supplier`.`s_suppkey`))))

which is directly an index lookup ("<primary_index_lookup>"), followed by an additional WHERE clause. So the overhead of full-blown subquery evaluation is
avoided. And this overhead is not neglectable, compared to the index lookup (assuming the relevant index pages are already in memory).

So the conclusion of my experiment is that unique_subquery is worth having. I'll have to direct simplification pulsions to some other code!

Note that there also exists a similar "index_subquery" optimization applying to non-unique indices. And it's worth having, for the same reasons.

mardi 4 octobre 2011

Optimizer tracing: how to configure it

In this blog post, my colleague Jørgen Løland described a new feature of MySQL 5.6: Optimizer Tracing. I recommend reading his article, as it presents this new feature in a simple, easy-to-read manner.

The Optimizer Tracing feature can help understanding what the Optimizer is doing; it is available since milestone 5.6.3, announced October 3rd at Oracle Open World (here is the changelog). It's good to see it mature now; I remember that Sergey Petrunia did the first prototype back in March 2009!

Today  I will be giving some must-have tips related to handling big traces.

First thing to know, a trace lives in main memory (internally it is allocated on the heap or free store of the MySQL Server). An SQL statement which gives the optimizer a lot of work (for example, by joining many tables) will generate a large trace. Up to gigabytes in some pathological cases! To avoid hogging memory then, a trace will never grow beyond the value of the @@optimizer_trace_max_mem_size session variable. Which has a default of 16 kilobytes; yes, it's a low value, to protect the innocent. I often raise it in my session, to 1 megabyte, which is enough for most queries in my daily work:

mysql> SET OPTIMIZER_TRACE_MAX_MEM_SIZE=1000000;

If a trace was truncated because exceeding this limit, the column INFORMATION_SCHEMA.OPTIMIZER_TRACE.MISSING_BYTES_BEYOND_MAX_MEM_SIZE shows a non-zero value: the number of bytes which could not be added to the trace.

When I read a trace I like to scroll up and down in it, search for some optimization phase (that means searching for some keyword in the trace); to do this, I set, in the "mysql" command-line client, the pager to "less" (I'm using Unix):

mysql> pager less;

This is very useful. I have all "less" commands under hand: can save to a file, can search for a regular expression match, search forward or backward... One last benefit is that when I have finished reading, quitting "less" makes the trace go away from the terminal, it does not linger on my screen forever, does not occupy the terminal's display history...

When there are many tables to join, as I said above the trace can grow a lot, because the Optimizer evaluates many possible join orders (using an algorithm known as "greedy search"): each evaluated join order is mentioned in the trace. Greedy search ends up being the greatest part of the trace. What if I want to see the trace of what happens in the Optimizer after greedy search is complete? If I set @@optimizer_trace_max_mem_size to a low value, it will trim greedy search and what follows. If I set @@optimizer_trace_max_mem_size to a high value, to see what I want, greedy search will be traced which will possibly exceed the amount of memory I can afford on this machine... It would be nice if I could tell the system: "please do not trace this and that". In my case, it would be "please do not trace greedy search".

As an example, let's consider twenty tables, t1...t20, all similar to this one:

mysql> CREATE TABLE t1 (a INT, INDEX(a)) ENGINE=MYISAM;
mysql> INSERT INTO t1 VALUES(x),(y); # x and y being some integers

and let's run this query, after turning tracing on:
mysql> SET OPTIMIZER_TRACE="ENABLED=ON,END_MARKER=ON";
mysql> EXPLAIN SELECT 1 FROM t1  JOIN t2 ON t2.a=t1.a JOIN t3 ON t3.a=t2.a JOIN t4 ON t4.a=t3.a JOIN t5 ON t5.a=t4.a JOIN t6 ON t6.a=t5.a JOIN t7 ON t7.a=t6.a JOIN t8 ON t8.a=t7.a JOIN t9 ON t9.a=t8.a JOIN t10 ON t10.a=t9.a JOIN t11 ON t11.a=t10.a JOIN t12 ON t12.a=t11.a JOIN t13 ON t13.a=t12.a JOIN t14 ON t14.a=t13.a JOIN t15 ON t15.a=t14.a JOIN t16 ON t16.a=t15.a JOIN t17 ON t17.a=t16.a JOIN t18 ON t18.a=t17.a JOIN t19 ON t19.a=t18.a JOIN t20 ON t20.a=t19.a;
+----+-------------+-------+-------+---------------+------+---------+------------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref        | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+------------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | a             | a    | 5       | NULL       |    2 | Using where; Using index |
|  1 | SIMPLE      | t2    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using where; Using index |
|  1 | SIMPLE      | t3    | ref   | a             | a    | 5       | test.t2.a  |    1 | Using where; Using index |
|  1 | SIMPLE      | t4    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using index              |
|  1 | SIMPLE      | t5    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using index              |
|  1 | SIMPLE      | t6    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using index              |
|  1 | SIMPLE      | t7    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using index              |
|  1 | SIMPLE      | t8    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using index              |
|  1 | SIMPLE      | t9    | ref   | a             | a    | 5       | test.t1.a  |    1 | Using where; Using index |
|  1 | SIMPLE      | t10   | ref   | a             | a    | 5       | test.t9.a  |    1 | Using where; Using index |
|  1 | SIMPLE      | t11   | ref   | a             | a    | 5       | test.t1.a  |    1 | Using where; Using index |
|  1 | SIMPLE      | t12   | ref   | a             | a    | 5       | test.t11.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t13   | ref   | a             | a    | 5       | test.t12.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t14   | ref   | a             | a    | 5       | test.t12.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t15   | ref   | a             | a    | 5       | test.t12.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t16   | ref   | a             | a    | 5       | test.t12.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t17   | ref   | a             | a    | 5       | test.t12.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t18   | ref   | a             | a    | 5       | test.t11.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t19   | ref   | a             | a    | 5       | test.t12.a |    1 | Using where; Using index |
|  1 | SIMPLE      | t20   | ref   | a             | a    | 5       | test.t11.a |    1 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+------------+------+--------------------------+
As optimizer developer, it catches my eye that some tables have "Using where" and others don't. "Using where" tells me that a condition is evaluated after fetching a row from the table, but does not tell me what condition. To know more, I will look at the trace. But I'm not interested in the trace of greedy search, which likely accounts, in this 20-table example, for most of the total trace, and would just make my reading less comfortable, or even hog memory, if I had more than those 20 tables or more indices on them, or more complex conditions. So I do:

mysql> SET OPTIMIZER_TRACE_FEATURES="GREEDY_SEARCH=OFF";
and run the EXPLAIN again. To show the resulting trace, I'm posting a screenshot of how it is displaid by the JsonView Firefox add-on. Using this add-on requires taking two precautions:
  • turning off the end_marker flag of @@optimizer_trace (this flag is good for human-readability but is not JSON-compliant)
  • sending the trace to a file without escaping of newlines (which is why I use INTO DUMPFILE instead of INTO OUTFILE):

mysql> SET OPTIMIZER_TRACE="END_MARKER=OFF";
mysql> SELECT TRACE INTO DUMPFILE "/tmp/trace.json" FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;

Here's the trace now in Firefox:

We see how greedy_search=off has eliminated the trace of greedy search ("considered_execution_plans"), replacing it with just an ellipsis ("...")! Then there is what I wanted to see: the part after greedy search, named "attached_conditions_summary", which describes what condition is behind each "Using where" in EXPLAIN. There are equality conditions of course. Some coming directly from the "JOIN ON" conditions. Some deduced by equality propagation (if t1.a=t2.a and t2.a=t3.a then t1.a=t3.a, a new condition which we see is attached to t3). There are also "IS NOT NULL" conditions; indeed, an equality condition of the form t1.a=t2.a allows us to deduce that both t1.a and t2.a are not NULL, so if rows of t1 are retrieved first they can be filtered with this deduced condition, known as early NULL filtering.

There are other flags in @@optimizer_trace_features; we added a flag for each feature which we think can make the trace grow unreasonably large:
  • greedy search
  • repeated execution of subqueries (one execution per row)
  • repeated range optimization (known as "range checked for each record" in EXPLAIN)
  • range optimization in general.
I think I have shown enough for today. A positive note to finish: in the Optimizer team, this tracing has already helped us to debug some problems, so has started to fulfill its goal!

For the curious: the Optimizer Trace is covered in full detail, including the above, and more, in a chapter of the "MySQL Internals" manual.

lundi 3 octobre 2011

Kick-off

Hi all!
Here is a new blog; I'll post here some thoughts, tutorials... all related to my job - software developer at Oracle Corporation, working on MySQL. It will soon be nine years since I joined MySQL AB. After working on interesting topics: Replication, Online Backup, the Maria storage engine, each of them having taught me something, I'm now full-time on the Optimizer, and guess what - it is an interesting topic and it teaches me something!