February 12, 2011 
Definition:
function pre_schema_upgrade() {}
Runs before the schema is upgraded.
Source code
function pre_schema_upgrade() {
global $wp_current_db_version, $wp_db_version, $wpdb;
// Upgrade versions prior to 2.9
if ( $wp_current_db_version < 11557 ) {
// Delete duplicate options. Keep the option with the highest option_id.
$wpdb->query("DELETE o1 FROM $wpdb->options AS o1 JOIN $wpdb->options AS o2 USING (`option_name`) WHERE o2.option_id > o1.option_id");
// Drop the old primary key and add the new.
$wpdb->query("ALTER TABLE $wpdb->options DROP PRIMARY KEY, ADD PRIMARY KEY(option_id)");
// Drop the old option_name index. dbDelta() doesn't do the drop.
$wpdb->query("ALTER TABLE $wpdb->options DROP INDEX option_name");
}
}
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