MySQL: Execute CREATE TEMPORARY TABLE SELECT just one time

if-not-existsMySQLtemporary-tables

I want to create a temporary table with some select-data.

My (strange) problem is that I have to do the same query more than one time.

CREATE TEMPORARY TABLE IF NOT EXISTS cache (id int(11) NOT NULL, INDEX (id)) 
SELECT id FROM table WHERE xyz;

CREATE TEMPORARY TABLE IF NOT EXISTS cache (id int(11) NOT NULL, INDEX (id)) 
SELECT id FROM table WHERE xyz;

Unfortunately, MySQL executes the second query although the temp-table already exists.

QUESTIONS

Is there a way to tell MySQL that the creation of the temporary table and the SELECT query are just executed if the table does not exist?
Maybe with an if-condition?
Or can I check to some information schema if a temp-table exists?
Thanks for some help!

Best regards,

Timo

Best Answer

Here is the nature of table creation with CREATE TEMPORARY TABLE

You can use the TEMPORARY keyword when creating a table. A TEMPORARY table is visible only to the current connection, and is dropped automatically when the connection is closed. This means that two different connections can use the same temporary table name without conflicting with each other or with an existing non-TEMPORARY table of the same name. (The existing table is hidden until the temporary table is dropped.) To create temporary tables, you must have the CREATE TEMPORARY TABLES privilege.

My guess would be you must have done the following sequence of events:

You ran CREATE TEMPORARY TABLE
Terminated DB Connection (which makes the temporary table drop immediately)
You had to run CREATE TEMPORARY TABLE again because he table did not exist

I have seen this problem victimize many developers, especially when it comes to MySQL Replication. For example, if you run STOP SLAVE;, the SQL Thread will close. Any tables create with CREATE TEMPORARY TABLE will be dropped. When you run START SLAVE;, MySQL Replication breaks instantly because the table needed for INSERTs, UPDATEs, or DELETEs no longer exist. I have had to perform mysqlbinlog dumps on relay logs to locate the original CREATE TEMPORARY TABLE statements, run them as CREATE TABLE in order for the table to remain accessible to other DB Connections, including the SQL Thread when START SLAVE; is reissued.

As for your Questions

Is there a way to tell MySQL that the creation of the temporary table and the SELECT query are just executed if the table does not exist?
Maybe with an if-condition?
Or can I check to some information schema if a temp-table exists?

You cannot check the information_schema for temp table existence. In the OS, there is no manifested .frm in datadir. You could find it in the folder defined in tmpdir.

Here is an example: I will create a temporary table called test.junk and try to locate the whereabouts of the table. (Note: I am using MySQL for Windows)

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.5.12-log MySQL Community Server (GPL)

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test
Database changed
mysql> create temporary table junk (a int) engine=MyISAM;
Query OK, 0 rows affected (0.02 sec)

mysql> show create table junk\G
*************************** 1. row ***************************
       Table: junk
Create Table: CREATE TEMPORARY TABLE `junk` (
  `a` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row in set (0.01 sec)

mysql> select count(1) from information_schema.tables
-> where table_schema='test' and table_name='junk';
+----------+
| count(1) |
+----------+
|        0 |
+----------+
1 row in set (0.08 sec)

mysql> show variables like 'datadir';
+---------------+-----------------------+
| Variable_name | Value                 |
+---------------+-----------------------+
| datadir       | C:\MySQL_5.5.12\data\ |
+---------------+-----------------------+
1 row in set (0.00 sec)

mysql> show variables like 'tmpdir';
+---------------+--------------------------------------+
| Variable_name | Value                                |
+---------------+--------------------------------------+
| tmpdir        | C:\Users\redwards\AppData\Local\Temp |
+---------------+--------------------------------------+
1 row in set (0.00 sec)

mysql>

The table is not visible in INFORMATION_SCHEMA.TABLES. Let's locate the table in the OS

C:\MySQL_5.5.12\data\test>cd
C:\MySQL_5.5.12\data\test

C:\MySQL_5.5.12\data\test>dir junk.*
 Volume in drive C has no label.
 Volume Serial Number is 2C92-485B

 Directory of C:\MySQL_5.5.12\data\test

File Not Found

C:\MySQL_5.5.12\data\test>dir C:\Users\redwards\AppData\Local\Temp\*.frm
 Volume in drive C has no label.
 Volume Serial Number is 2C92-485B

 Directory of C:\Users\redwards\AppData\Local\Temp

10/19/2012  11:04 AM             8,554 #sql7ac_3_0.frm
               1 File(s)          8,554 bytes
               0 Dir(s)  190,200,049,664 bytes free

C:\MySQL_5.5.12\data\test>dir C:\Users\redwards\AppData\Local\Temp\*.MY*
 Volume in drive C has no label.
 Volume Serial Number is 2C92-485B

 Directory of C:\Users\redwards\AppData\Local\Temp

10/19/2012  11:04 AM                 0 #sql7ac_3_0.MYD
10/19/2012  11:04 AM             1,024 #sql7ac_3_0.MYI
               2 File(s)          1,024 bytes
               0 Dir(s)  190,200,049,664 bytes free

C:\MySQL_5.5.12\data\test>

There is the temp table, in the tmpdir folder. When I closed the mysql client and run dir C:\Users\redwards\AppData\Local\Temp\*.frm, the table vanished.

RECOMMENDATION

You can do one of two things

Create the temp table, load it with data, keep the same DB Connection around, and then access the data with the same ProcessID.
Do CREATE TEMPORARY TABLE IF NOT EXISTS (This is really a band-aid to improper design but may circumvent the problem).

UPDATE 2012-12-18 13:06 EDT

Further Observation #1

If you stayed with the same DB Connection and still have this problem, here something else not previously mentioned: You gave the temp table an index. That should be a UNIQUE INDEX or PRIMARY KEY.

Further Observation #2

You could shift the operation from a CREATE TABLE ... SELECT to CREATE TABLE ... INSERT IGNORE

Further Observation #3

If you want to use temp tables, please specify MyISAM. InnoDB has a tendency to leave pigeon holes in the data dictionary with ibdata1 in the event of a crash or upon DB Connection termination.

With these three(3) observations in mind, here is my recommended change

CREATE TEMPORARY TABLE IF NOT EXISTS cache
(id int(11) NOT NULL, PRIMARY KEY (id)) ENGINE=MyISAM;
INSERT IGNORE INTO cache SELECT id FROM table WHERE xyz;

CREATE TEMPORARY TABLE IF NOT EXISTS cache
(id int(11) NOT NULL, PRIMARY KEY (id)) ENGINE=MyISAM;
INSERT IGNORE INTO cache SELECT id FROM table WHERE xyz;

Give it a Try !!!

Related Solutions

SQL Server 2005 – INSERT Performance: Temporary Tables vs Table Variables

The obvious difference between the two plans is that the fast one is parallel and the slower one serial.

This is one of the limitations of plans that insert into table variables. As mentioned in the comments (and it seems as though it had the desired effect) you could try doing

INSERT INTO @DATA ( ... ) 
EXEC('SELECT .. FROM ...')

to see if that gets around the limitation.

MySQL optimization – year column grouping – using temporary table, filesort

I don't see a lot of opportunity for improvement.

The index you added was probably a big help, because it's being used for the range matching on the WHERE clause (type => range, key => tran_date), and it's being used as a covering index (extra => using index), avoiding the need to seek into the table to fetch the row data.

But since you're using functions to construct the financial_year value for the group by, both the "using filesort" and "using temporary" can't be avoided. But, those aren't the real problem. The real problem is that you're evaluating MONTH(tran_date) 346,485 times and YEAR(tran_date) at least that many times... ~700,000 function calls in one second doesn't seem too bad.

Plan B: I am definitely not a fan of storing redundant data, and I'm dead-set against making the application responsible for maintaining it... but one option I might be tempted to try would be to create a dashboard_stats_by_financial_year table, and use after-insert/update/delete triggers on the transactions1 table to manage keeping those stats current.

That option has a cost, of course -- adding to the amount of time it takes to update/insert/delete a transaction... but, waiting > 1200 milliseconds for stats for your dashboard is a cost, too. So it may come down to whether you want to pay for it now or pay for it later.