Source

File: wp-includes/functions.php

function is_blog_installed() {
	global $wpdb;

	/*
	 * Check cache first. If options table goes away and we have true
	 * cached, oh well.
	 */
	if ( wp_cache_get( 'is_blog_installed' ) ) {
		return true;
	}

	$suppress = $wpdb->suppress_errors();
	if ( ! wp_installing() ) {
		$alloptions = wp_load_alloptions();
	}
	// If siteurl is not set to autoload, check it specifically.
	if ( ! isset( $alloptions['siteurl'] ) ) {
		$installed = $wpdb->get_var( "SELECT option_value FROM $wpdb->options WHERE option_name = 'siteurl'" );
	} else {
		$installed = $alloptions['siteurl'];
	}
	$wpdb->suppress_errors( $suppress );

	$installed = ! empty( $installed );
	wp_cache_set( 'is_blog_installed', $installed );

	if ( $installed ) {
		return true;
	}

	// If visiting repair.php, return true and let it take over.
	if ( defined( 'WP_REPAIRING' ) ) {
		return true;
	}

	$suppress = $wpdb->suppress_errors();

	/*
	 * Loop over the WP tables. If none exist, then scratch installation is allowed.
	 * If one or more exist, suggest table repair since we got here because the
	 * options table could not be accessed.
	 */
	$wp_tables = $wpdb->tables();
	foreach ( $wp_tables as $table ) {
		// The existence of custom user tables shouldn't suggest an insane state or prevent a clean installation.
		if ( defined( 'CUSTOM_USER_TABLE' ) && CUSTOM_USER_TABLE == $table ) {
			continue;
		}
		if ( defined( 'CUSTOM_USER_META_TABLE' ) && CUSTOM_USER_META_TABLE == $table ) {
			continue;
		}

		if ( ! $wpdb->get_results( "DESCRIBE $table;" ) ) {
			continue;
		}

		// One or more tables exist. We are insane.

		wp_load_translations_early();

		// Die with a DB error.
		$wpdb->error = sprintf(
			/* translators: %s: Database repair URL. */
			__( 'One or more database tables are unavailable. The database may need to be <a href="%s">repaired</a>.' ),
			'maint/repair.php?referrer=is_blog_installed'
		);

		dead_db();
	}

	$wpdb->suppress_errors( $suppress );

	wp_cache_set( 'is_blog_installed', false );

	return false;
}