Thursday, April 29, 2010

Exchanging partitions with tables

MySQL PartitionsWhile I was presenting my partitioning tutorial at the latest MySQL Conference, I announced a new feature that was, as far as I knew, still in the planning stage. Mattias Jonsson, one of the partitions developers, was in attendance, and corrected me, explaining that the feature was actually available in a prototype.

So, we can have a look at this improvement, which I am sure will make DBAs quite happy. The new feature is an instantaneous exchange between a partition and a table with the same structure. Using this feature, you can transfer the contents of one partition to one table, and vice versa. Since the transition is done only in the attribution of the data, there is no copy involved. The data stays where it is at the moment. What is in the table ends up in the partition and what's in the partition ends up in the table. Let's see an example.

With the data in figure 1, where we have a partitioned table t1 and an empty table t2 with the same structure, we can issue the following statement:
ALTER TABLE t1
EXCHANGE PARTITION p2
WITH TABLE t2


After the exchange, partition p2 is empty, and table t2 contains 4 records.
If we repeat the command, the contents will be swapped again, leaving table t2 empty and partition p2 with its original contents.

If you want to test on your own, you can get the code from Launchpad. Once you get the code, you can use cmake to build the server.

$ cmake-gui .
# add the options you need. For example, enable innodb
# or else you will need to load it as a plugin.
$ make && ./scripts/make_binary_distribution

You can then use this script to test the new functionality. You may want to change Innodb with MyISAM to test it thoroughly. At the moment, it doesn't work with the archive engine (yet). UPDATE 2010-04-30 Now it does! Mattias has fixed the bug.

# ############################
# test_exchange_partitions.sql
# ############################
use test;
set default_storage_engine=innodb;
drop procedure if exists compare_tables;
delimiter //
create procedure compare_tables (wanted int)
reads sql data
begin
set @part_table := (select count(*) from t1);
set @non_part_table := (select count(*) from t2);
select @part_table, @non_part_table,
if(@non_part_table = wanted, "OK", "error") as expected;
end //
delimiter ;

drop table if exists t1, t2;
create table t1 (i int) # not null primary key)
partition by range (i)
(
partition p01 values less than (100001),
partition p02 values less than (200001),
partition p03 values less than (300001),
partition p04 values less than (400001),
partition p05 values less than (500001),
partition p06 values less than (600001),
partition p07 values less than (700001),
partition p08 values less than (800001),
partition p09 values less than (900001),
partition p10 values less than (1000001),
partition p11 values less than (maxvalue));

create table t2 (i int ) ; # not null primary key);

select table_name, engine
from information_schema.tables
where table_schema='test' and table_type='base table';


select 'generating 1 million records. ...' as info;
# generates 1 million records
# see this article for details
# http://datacharmer.blogspot.com/2007/12/data-from-nothing-solution-to-pop-quiz.html
create or replace view v3 as select null union all select null union all select null;
create or replace view v10 as select null from v3 a, v3 b union all select null;
create or replace view v1000 as select null from v10 a, v10 b, v10 c;
set @n = 0;
insert into t1 select @n:=@n+1 from v1000 a,v1000 b;

select partition_name, table_rows from information_schema . partitions where table_name='t1' and table_schema='test';

call compare_tables(0);

alter table t1 exchange partition p04 with table t2;
call compare_tables(100000);

select partition_name, table_rows from information_schema . partitions where table_name='t1' and table_schema='test';

alter table t1 exchange partition p04 with table t2;
call compare_tables(0);

alter table t1 exchange partition p04 with table t2;
call compare_tables(100000);

Here is a test run:

$ ~/sandboxes/msb_5_6_99/use -t test -vvv < test_exch_part.sql
--------------
set default_storage_engine=innodb
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
drop procedure if exists compare_tables
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
create procedure compare_tables (wanted int)
reads sql data
begin
set @part_table := (select count(*) from t1);
set @non_part_table := (select count(*) from t2);
select @part_table, @non_part_table,
if(@non_part_table = wanted, "OK", "error") as expected;
end
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
drop table if exists t1, t2
--------------

Query OK, 0 rows affected (0.07 sec)

--------------
create table t1 (i int)
partition by range (i)
(
partition p01 values less than (100001),
partition p02 values less than (200001),
partition p03 values less than (300001),
partition p04 values less than (400001),
partition p05 values less than (500001),
partition p06 values less than (600001),
partition p07 values less than (700001),
partition p08 values less than (800001),
partition p09 values less than (900001),
partition p10 values less than (1000001),
partition p11 values less than (maxvalue))
--------------

Query OK, 0 rows affected (0.08 sec)

--------------
create table t2 (i int )
--------------

Query OK, 0 rows affected (0.14 sec)

--------------
select table_name, engine
from information_schema.tables
where table_schema='test' and table_type='base table'
--------------

+------------+--------+
| table_name | engine |
+------------+--------+
| t1 | InnoDB |
| t2 | InnoDB |
+------------+--------+
2 rows in set (0.01 sec)

--------------
select 'generating 1 million records. ...' as info
--------------

+-----------------------------------+
| info |
+-----------------------------------+
| generating 1 million records. ... |
+-----------------------------------+
1 row in set (0.00 sec)

--------------
create or replace view v3 as select null union all select null union all select null
--------------

Query OK, 0 rows affected (0.12 sec)

--------------
create or replace view v10 as select null from v3 a, v3 b union all select null
--------------

Query OK, 0 rows affected (0.14 sec)

--------------
create or replace view v1000 as select null from v10 a, v10 b, v10 c
--------------

Query OK, 0 rows affected (0.09 sec)

--------------
set @n = 0
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
insert into t1 select @n:=@n+1 from v1000 a,v1000 b
--------------

Query OK, 1000000 rows affected (10.01 sec)
Records: 1000000 Duplicates: 0 Warnings: 0

--------------
select partition_name, table_rows from information_schema . partitions where table_name='t1' and table_schema='test'
--------------

+----------------+------------+
| partition_name | table_rows |
+----------------+------------+
| p01 | 100623 |
| p02 | 100623 |
| p03 | 100623 |
| p04 | 100623 |
| p05 | 100623 |
| p06 | 100623 |
| p07 | 100623 |
| p08 | 100623 |
| p09 | 100623 |
| p10 | 100623 |
| p11 | 0 |
+----------------+------------+
11 rows in set (0.01 sec)

--------------
call compare_tables(0)
--------------

+-------------+-----------------+----------+
| @part_table | @non_part_table | expected |
+-------------+-----------------+----------+
| 1000000 | 0 | OK |
+-------------+-----------------+----------+
1 row in set (0.56 sec)

Query OK, 0 rows affected (0.56 sec)

--------------
alter table t1 exchange partition p04 with table t2
--------------

Query OK, 0 rows affected (0.01 sec)

--------------
call compare_tables(100000)
--------------

+-------------+-----------------+----------+
| @part_table | @non_part_table | expected |
+-------------+-----------------+----------+
| 900000 | 100000 | OK |
+-------------+-----------------+----------+
1 row in set (0.54 sec)

Query OK, 0 rows affected (0.54 sec)

--------------
select partition_name, table_rows from information_schema . partitions where table_name='t1' and table_schema='test'
--------------

+----------------+------------+
| partition_name | table_rows |
+----------------+------------+
| p01 | 100623 |
| p02 | 100623 |
| p03 | 100623 |
| p04 | 0 |
| p05 | 100623 |
| p06 | 100623 |
| p07 | 100623 |
| p08 | 100623 |
| p09 | 100623 |
| p10 | 91799 |
| p11 | 0 |
+----------------+------------+
11 rows in set (0.01 sec)

--------------
alter table t1 exchange partition p04 with table t2
--------------

Query OK, 0 rows affected (0.05 sec)

--------------
call compare_tables(0)
--------------

+-------------+-----------------+----------+
| @part_table | @non_part_table | expected |
+-------------+-----------------+----------+
| 1000000 | 0 | OK |
+-------------+-----------------+----------+
1 row in set (0.56 sec)

Query OK, 0 rows affected (0.56 sec)

--------------
alter table t1 exchange partition p04 with table t2
--------------

Query OK, 0 rows affected (0.00 sec)

--------------
call compare_tables(100000)
--------------

+-------------+-----------------+----------+
| @part_table | @non_part_table | expected |
+-------------+-----------------+----------+
| 900000 | 100000 | OK |
+-------------+-----------------+----------+
1 row in set (0.56 sec)

Query OK, 0 rows affected (0.56 sec)

Bye

Notice that the value for "table_rows" is only approximate with InnoDB, while it is reliable for MyISAM. Anyway, when it says that a partition has 0 records, it's reliable for any engine. Here you see that, after the exchange, partition p04 is empty.
The exchange is repeated twice, to make sure that it works both ways.
Notice also that, if the table contains data that doesn't fit with the partition, the server throws an error, and the exchange does not happen.

mysql > insert into t2 values (2000000);
Query OK, 1 row affected (0.00 sec)

mysql > alter table t1 exchange partition p04 with table t2;
ERROR 1697 (HY000): Found row that does not match the partition

If you remove the offending row from the table, the exchange works as expected.

6 comments:

Swany said...

Will you eventually be able to swap a partition in one table with a partition in another, without having to swap through an empty table in-between?

Mattias said...

@Swany: There is no plans for that currently, and not for EXPORT/IMPORT PARTITION either, since EXCHANGE PARTITION WITH TABLE enhances the current ALTER statement(s) to perform all these operations. But you can always report it as a feature request at bugs.mysql.com.

Mark Callaghan said...

This is a great feature to have.

mat said...

What happens if there is an (global) index on the partitioned table (if possible at all)?
Thanks

datacharmer said...

@mat,
The index is partitioned. This feature takes data and index and convert them from table to partition instantly, without need for further elaboration. At least, in the tests that I have done so far, it seems to work as advertised.

mat said...

Is it possibile to have a nonpartitioned index on a partitioned table? I Know, I should read the manual...

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