Question

This is a weird one. I am trying to use Views in MySQL (I’m reasonably new to MySQL with more experience with Sybase and SQL Server). Any way this new project we are using MySQL as it seems to have good performance. However to make querying for a web front end simpler we decided to create a few views, all work well, but they take forever to run.

The views are very simple, just select statements (these tables do have a few million rows in them). Say for example this query:

SELECT CAST(classifier_results.msgDate as DATE) AS mdate
       ,classifier_results.objClass AS objClass
       ,COUNT(classifier_results.objClass) AS obj
       ,classifier_results.subjClass AS subjClass
       ,COUNT(classifier_results.subjClass) AS subj 
FROM classifier_results 
WHERE (classifier_results.msgDate >= (curdate() - 20)) 
GROUP BY 
  CAST(classifier_results.msgDate as DATE)
  ,classifier_results.objClass
  ,classifier_results.subjClass 
ORDER BY classifier_results.msgDate DESC

When run as a normal select takes around 1.5 seconds to return a result.

However when this query is put into a view (as is) – i.e.

CREATE VIEW  V1a_sentiment_AI_current AS    
SELECT CAST(classifier_results.msgDate as DATE) AS mdate
       ,classifier_results.objClass AS objClass
       ,COUNT(classifier_results.objClass) AS obj
       ,classifier_results.subjClass AS subjClass
       ,COUNT(classifier_results.subjClass) AS subj 
FROM classifier_results 
WHERE (classifier_results.msgDate >= (curdate() - 20)) 
GROUP BY 
  CAST(classifier_results.msgDate as DATE)
  ,classifier_results.objClass
  ,classifier_results.subjClass 
ORDER BY classifier_results.msgDate DESC

The query takes about 10 times longer (22-30 seconds). So I’m thinking maybe there is some optimization or query caching that doesnt work with Views or maybe there is some setting we’ve missed in the MySQL config. But is there any way to speed up this view so its just a nice placeholder for this query?

Running EXPLAIN on the two queries:
The normal select gives:

1, SIMPLE, classifier_results, ALL, idx_date, , , , 594845, Using where; Using temporary; Using filesort

The view select gives:

1, PRIMARY, , ALL, , , , , 100,
2, DERIVED, classifier_results, ALL, idx_date, , , , 594845, Using where; Using temporary; Using filesort

Answer 1

Try re-creating your view using this:

CREATE ALGORITHM = MERGE VIEW `V1a_sentiment_AI_current` AS    
SELECT CAST(classifier_results.msgDate as DATE) AS mdate
   ,classifier_results.objClass AS objClass
   ,COUNT(classifier_results.objClass) AS obj
   ,classifier_results.subjClass AS subjClass
   ,COUNT(classifier_results.subjClass) AS subj 
FROM classifier_results 
WHERE (classifier_results.msgDate >= (curdate() - 20)) 
GROUP BY 
  CAST(classifier_results.msgDate as DATE)
  ,classifier_results.objClass
  ,classifier_results.subjClass 
ORDER BY classifier_results.msgDate DESC

More information on MySQL’s view processing algorithms can be found here.