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.
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 do not want to use a TEMPORARY
table, you should make sure that
each invocation of the script creates a uniquely named table and that
it drops that table when it is no longer needed. To accomplish 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:
my $tbl_name = "tmp_tbl_$$";
tbl_name = "tmp_tbl_" + Process.pid.to_s
$tbl_name = "tmp_tbl_" . posix_getpid ();
import os tbl_name = "tmp_tbl_%d" % os.getpid ()
Note that even if you create a table name using a value such as a PID that is guaranteed to be unique to a given script invocation, there may still be a chance that the table will already 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 EXISTStbl_name
Then you can go ahead and create the new table.
Connection identifiers are another source of unique values. The MySQL server reuses these numbers over time, but no two simultaneous connections to the server have the same ID. To get your connection ID, execute this statement, and retrieve the result:
SELECT CONNECTION_ID();
Some MySQL APIs expose the connection ID directly without
requiring any statement to be executed. For example, in Perl DBI, use
the
mysql_thread_id
attribute of your database handle:
my $tbl_name = "tmp_tbl_" . $dbh->{mysql_thread_id};
tbl_name = "tmp_tbl_" + dbh.func(:thread_id).to_s
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