MySQL - Why specific query doesn't use cache?

Question

My customer is running MySQL 5.7.16 on Ubuntu16.04 as backend for Wordpress site. A lot of heavy, bad written SQLs. I was able to improve situation significant by enabling MySQL cache, slow query log is almost empty now. But - certain query doesn't use cache and I cannot understand why. SQL uses two tables, which are pretty static, so it's not cache invalidation. Any idea how I can debug that?

TIA, Vitaly

1) SHOW STATUS LIKE "qcache%";
+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_free_blocks      | 1700     |
| Qcache_free_memory      | 49555584 |
| Qcache_hits             | 17960465 |
| Qcache_inserts          | 6928571  |
| Qcache_lowmem_prunes    | 1850967  |
| Qcache_not_cached       | 924283   |
| Qcache_queries_in_cache | 13303    |
| Qcache_total_blocks     | 28441    |
+-------------------------+----------+

2)Example of SQL:

SELECT SQL_CALC_FOUND_ROWS  wp_posts.ID 
FROM wp_posts  
  LEFT JOIN wp_postmeta 
    ON ( wp_posts.ID = wp_postmeta.post_id )  
  LEFT JOIN wp_postmeta AS mt1 
    ON (wp_posts.ID = mt1.post_id 
    AND mt1.meta_key = '_pagefrog_fbia_status' ) 
WHERE 1=1  
  AND (     ( wp_postmeta.meta_key = '_pagefrog_fbia_status' 
          AND wp_postmeta.meta_value = '1' )   
       OR     mt1.post_id IS NULL   
       OR   ( wp_postmeta.meta_key = '_pagefrog_fbia_status' 
          AND wp_postmeta.meta_value = '' ) ) 
  AND wp_posts.post_type = 'post' 
  AND ((wp_posts.post_status = 'publish')) 
GROUP BY wp_posts.ID 
ORDER BY wp_posts.post_date DESC 
LIMIT 0, 10

Addenda

SHOW VARIABLES LIKE '%query_cache%';
+------------------------------+----------+
| Variable_name | Value |
+------------------------------+----------+
| have_query_cache | YES |
| query_cache_limit | 1048576 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 83886080 |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |    
+-----------------------------+----------+

Answer 1

• When either table is modified, all entries in the QC are removed (pruned) from the QC for that table.
• If the query has too big a resultset, it won't go into the QC. (Please provide SHOW VARIABLES LIKE 'query_cache*';

Do you have this on wp_postmeta: PRIMARY KEY(post_ID, meta_key) ? If you can switch to that, performance should improve when the QC is not used. If you have trouble, let's see SHOW CREATE TABLE wp_postmeta; some WP versions are really lame when it comes to optimizing that schema.

Do you need the GROUP BY? JOIN...GROUP BY often suffers from the explosion-implosion syndrome that makes such queries slow. There may be a workaround.

Will you be "paginating"? If so, use of OFFSET is inefficient.

Getting rid of SQL_CALC_FOUND_ROWS will speed up the query.

That is a confusing query, what is it trying to do?

Index

PRIMARY KEY (meta_id),         -- useless and slows things down
KEY post_id (post_id),         -- inadequate
KEY meta_key (meta_key(191))   -- prefixing is next to useless

Change to this

Change `meta_key` from VARCHAR(255) to VARCHAR(191)
PRIMARY KEY (post_id, meta_key)
no other indexes unless you ever search for meta_key without post_id

Notes:

• You probably don't have very long keys, so changing to 191 may be safe. You could run SELECT MAX(CHAR_LENGTH(meta_key)) FROM .. to see what the current max is.
• If you can't shrink to 191, then try to upgrade to 5.6.3 / 5.5.14, which allows you to increase the limit (with some effort), or 5.7.7, which allows it by default.
• 'Composite' keys are often better than single-column keys, especially in this case.
• Having the primary access pattern be the PK makes it faster.
• These improvements may not help that particular contorted query, but it will help the table in other ways.

(I have been ranting about WP for years. I would be happy to advise them.)

Answer 2

Thanks to Rick for many interesting ideas. And yes - "Getting rid of SQL_CALC_FOUND_ROWS will speed up the query." answers my question. It seems that with SQL_CALC_FOUND_ROWS cache should be bigger for save intermediate result.

Answer 3

query_cache_min_res_unit = 512 in your .cfg/.ini would allow more results to be in QC. You likely have many results smaller than 4,096 result size, meaning you have still reserved 4,096 for your smallest queries reducing the capacity of query_cache_size by (4096-512)=3584 that would still be available for other results.