Generating Unique Table Names

3.27.1 Problem

You need to create a table with a name that is guaranteed not to exist already.

3.27.2 Solution

If you can create a TEMPORARY table, it doesn't matter if the name exists already. Otherwise, try to generate a value that is unique to your client program and incorporate it into the table name.

3.27.3 Discussion

MySQL is a multiple-client database server, so if a given script that creates a transient table might be invoked by several clients simultaneously, you must take care to keep multiple invocations of the script from fighting over the same table name. If the script creates tables using CREATE TEMPORARY TABLE, there is no problem because different clients can create temporary tables having the same name without clashing.

If you can't use CREATE TEMPORARY TABLE because the server version is older than 3.23.2, you should make sure that each invocation of the script creates a uniquely named table. To do this, incorporate into the name some value that is guaranteed to be unique per invocation. A timestamp won't work, because it's easily possible for two instances of a script to be invoked within the same second. A random number may be somewhat better. For example, in Java, you can use the java.util.Random( ) class to create a table name like this:

import java.util.Random; import java.lang.Math; Random rand = new Random ( ); int n = rand.nextInt ( ); // generate random number n = Math.abs (n); // take absolute value String tblName = "tmp_tbl_" + n;

Unfortunately, random numbers only reduce the possibility of name clashes, they do not eliminate it. Process ID (PID) values are a better source of unique values. PIDs are reused over time, but never for two processes at the same time, so a given PID is guaranteed to be unique among the set of currently executing processes. You can use this fact to create unique table names as follows:

Perl:

my $tbl_name = "tmp_tbl_$$";

PHP:

$tbl_name = "tmp_tbl_" . posix_getpid ( );

Python:

import os tbl_name = "tmp_tbl_%d" % os.getpid ( )

Note that even if you create a table name using a value like a PID that is guaranteed to be unique to a given script invocation, there may still be a chance that the table will exist. This can happen if a previous invocation of the script with the same PID created a table with the same name, but crashed before removing the table. On the other hand, any such table cannot still be in use because it will have been created by a process that is no longer running. Under these circumstances, it's safe to remove the table if it does exist by issuing the following statement:

DROP TABLE IF EXISTS tbl_name

Then you can go ahead and create the new table.