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Search Results for: wrong index

How to find wrong indexing with glance view

Quite common beginners mistake is not to understand how indexing works and so index all columns used in the queries…. separately. So you end up with table which has say 20 indexes but all single …

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Indexing 101: Optimizing MySQL queries on a single table

I have recently seen several cases when performance for MySQL queries on a single table was terrible. The reason was simple: the wrong indexes were added and so the execution plan was poor. Here are …

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When EXPLAIN estimates can go wrong!

I have been working with a few customer cases and one interesting case popped up. The customer was facing a peculiar problem where the rows column in the EXPLAIN output of the query was totally …

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Joining on range? Wrong!

The problem I am going to describe is likely to be around since the very beginning of MySQL, however unless you carefully analyse and profile your queries, it might easily go unnoticed. I used it …

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Multi Column indexes vs Index Merge

The mistake I commonly see among MySQL users is how indexes are created. Quite commonly people just index individual columns as they are referenced in where clause thinking this is the optimal indexing strategy. For …

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3 ways MySQL uses indexes

I often see people confuse different ways MySQL can use indexing, getting wrong ideas on what query performance they should expect. There are 3 main ways how MySQL can use the indexes for query execution, …

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Multiple column index vs multiple indexes

(There is an updated version of the content in this post by Percona’s Stephane Combaudon available here.) After my previous post there were questions raised about Index Merge on Multiple Indexes vs Two Column Index …

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Should you name indexes while doing ALTER TABLE ?

MySQL Server does not require you to specify name of the index if you’re running ALTER TABLE statement – it is optional. Though what might be good practical reasons to specify the key name or …

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Why Index could refuse to work ?

Have you ever seen index which refused to be used even if there is every reason for it to work (from the glance view):

Why on the earth index would not be used you …

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Wrong GROUP BY makes your queries fragile

This is probably well known issue for everyone having some MySQL experience or experience with any other SQL database. Still I see this problem in many production applications so it is worth to mention it, …

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