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used_key_parts: EXPLAIN FORMAT=JSON provides insight into which part of multiple-column key is used

 | December 17, 2015 |  Posted In: EXPLAIN, EXPLAIN FORMAT=JSON is cool!, JSON, MySQL

In the previous post for this ongoing “EXPLAIN FORMAT=JSON is Cool!” series, we discussed covered indexes and how the  used_columns  array can help to choose them wisely. There is one more type of multiple-column indexes: composite indexes. Composite indexes are just indexes on multiple columns. Covered indexes are a subgroup of the larger set “composite indexes.” In this post we’ll […]

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used_columns: EXPLAIN FORMAT=JSON tells when you should use covered indexes

 | December 14, 2015 |  Posted In: EXPLAIN, EXPLAIN FORMAT=JSON is cool!, JSON, MySQL, MySQL 101

In the “MySQL Query tuning 101” video, Alexander Rubin provides an excellent example of when to use a covered index. On slide 25, he takes the query select name from City where CountryCode = ’USA’ and District = ’Alaska’ and population > 10000 and adds the index cov1(CountryCode, District, population, name) on table City. With Alex’s query tuning experience, making the right index decision is simple – but what about us mere mortals? If a query is more […]

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