postinst is in libpam-mysql 0.7~RC1-4ubuntu2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8  | #!/bin/sh
. /usr/share/debconf/confmodule
if [ -e /etc/pam-mysql.conf ] ; then
	db_get pam-mysql/config_file_noread
	if [ "$RET" = "true" ]; then chmod 0600 /etc/pam-mysql.conf; fi
fi
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