Filters `query` to fix slow queries during `post_exists()`

wp-importer-performance-fix.php

<?php

/**
 * This adds `post_type` and `post_status` to the query that happens during `post_exists()` so it can utilize the `type_status_date` index on `wp_posts`.
 * Hooked on `query` during `wp_import_post_data_raw` and removed during `wp_import_existing_post`.
 * 
 * Modifies a query like this from `post_exists()`:
 * SELECT ID FROM wp_posts WHERE 1=1 AND post_date = '2020-07-01 09:30:00' AND post_title = 'Some Post Title'
 * 
 * To this:
 * SELECT ID FROM wp_posts WHERE 1=1 AND post_date = '2020-07-01 09:30:00' AND post_title = 'Some Post Title' AND post_type = 'post' AND post_status = 'publish'
 * 
 * @param string $query
 * @return string
 */
function wpcomvip_import_query_filter ( $query ) {
    global $wp_import_post_data_raw;

    if ( is_null( $wp_import_post_data_raw ) || strpos( $query, 'SHOW FULL COLUMNS' ) !== false ) {
        return $query;
    }

    $modified_query_parts = array();
    if ( isset( $wp_import_post_data_raw['post_type'] ) ) {
        $modified_query_parts[] = "AND post_type = '" . $wp_import_post_data_raw['post_type'] . "'";
    }

    if ( isset( $wp_import_post_data_raw['status'] ) ) {
        $modified_query_parts[] = "AND post_status = '" . $wp_import_post_data_raw['status'] . "'";
    }

    $modified_query = $query . ' ' . implode( ' ', $modified_query_parts );

    $wp_import_post_data_raw = null;

    return $modified_query;
}

// Add our `query` filter after we get the post data we're trying to import.
add_filter( 'wp_import_post_data_raw', function( $post ) {

    // Set up global data to use in the `query` filter.
    global $wp_import_post_data_raw;
    $wp_import_post_data_raw = $post;

    // Add the `query` filter if it hasn't already been added AND the `post_type` is not `nav_menu_item`.
    if ( false === has_filter( 'query', 'wpcomvip_import_query_filter' ) && 'nav_menu_item' !== $post['post_type'] ) {
        add_filter( 'query', 'wpcomvip_import_query_filter' );
    }

    // return the unmodified data
    return $post;
} );

// Remove our `query` filter after we've done `post_exists()`.
add_filter( 'wp_import_existing_post', function( $post ) {

    // Remove the `query` filter.
    remove_filter( 'query', 'wpcomvip_import_query_filter' );

    // return the unmodified data
    return $post;
} );