Useful as part of a nest of toggles as to whether a CPT should be defined. For example, whether the theme supports it, or whether something filters it active, or ~this~.

does_db_have_cpt.php

<?php

function get_all_cpts_in_db() {
	global $wpdb;
	if ( false === ( $cpts = get_transient( 'all_cpts_in_db' ) ) {
		$cpts = $wpdb->get_col( "SELECT DISTINCT( `post_type` ) FROM {$wpdb->posts}" );
		set_transient( 'all_cpts_in_db', $cpts );
	}
	return (array) $cpts;
}

function does_db_have_cpt( $cpt_slug ) {
	$cpts = get_all_cpts_in_db();
	return in_array( $cpt_slug, $cpts );
}

add_action( 'save_post', 'maybe_purge_cpts_in_db_transient' );
function maybe_purge_cpts_in_db_transient( $post_id ) {
	$post_type = get_post_type( $post_id );
	if ( ! does_db_have_cpt( $post_type ) ) {
		delete_transient( 'all_cpts_in_db' );
	}
}