Test script for https://core.trac.wordpress.org/ticket/21760

21760.php

<?php
# for((i=100;i--;)); do php 21760.php; done
include __DIR__ . '/wp-load.php';

global $wpdb;
$memory = memory_get_usage();
$num_queries = $wpdb->num_queries;
$start = microtime( true );

get_term_by( 'slug', 'tag-117', 'post_tag' );
get_term_by( 'slug', 'tag-117', 'post_tag' );
get_term_by( 'slug', 'tag-117', 'post_tag' );
get_term_by( 'slug', 'tag-117', 'post_tag' );
get_term_by( 'slug', 'tag-117', 'post_tag' );
get_term_by( 'slug', 'tag-117', 'post_tag' );

get_term_by( 'name', 'Tag 117', 'post_tag' );
get_term_by( 'name', 'Tag 117', 'post_tag' );
get_term_by( 'name', 'Tag 117', 'post_tag' );
get_term_by( 'name', 'Tag 117', 'post_tag' );
get_term_by( 'name', 'Tag 117', 'post_tag' );
get_term_by( 'name', 'Tag 117', 'post_tag' );

get_term_by( 'name', 'Category 106', 'category' );
get_term_by( 'name', 'Category 106', 'category' );
get_term_by( 'name', 'Category 106', 'category' );
get_term_by( 'name', 'Category 106', 'category' );
get_term_by( 'name', 'Category 106', 'category' );
get_term_by( 'name', 'Category 106', 'category' );

get_term_by( 'id', 107, 'category' );
get_term_by( 'id', 107, 'category' );
get_term_by( 'id', 107, 'category' );
get_term_by( 'id', 107, 'category' );
get_term_by( 'id', 107, 'category' );
get_term_by( 'id', 107, 'category' );

$end = microtime( true );
echo ( $end - $start ) . "\t";
echo ( $wpdb->num_queries - $num_queries ) . "\t";
echo ( ( memory_get_usage() - $memory ) / 1024 / 1024 ) . "\t\n";