Why is my SQL query not using the table's composite index?
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I have a users table with the columns:
id (primary key),
(external_id, external_type, type)
And a settings table with the columns:
I execute the query:
SELECT users.id, users.type, users.external_id, users.created_at, users.updated_at, settings.id, settings.settings_id, settings.name, settings.value, settings.created_at, settings.updated_at, settings.type FROM users LEFT OUTER JOIN settings on settings.user_id = users.id WHERE users.external_id=3 and users.external_type="Owner"
In the Explain report, I see that:
- For the users table, the (external_id, external_type, type) index was identified as a possible key, but NOT used
- The settings table uses the (user_id, name) index
- I want to optimize this query
- So I want to get the users table to use the (external_id, external_type, type) composite index
Things I’ve done to debug:
- If I change the first line of the SELECT statement to remove users.created_at, users.updated_at, it uses the index
- If I try adding a (external_id, external_type) non-unique index to the users table, it still doesn’t use it
- If I change the query’s WHERE clause to add and users.type="Blah", it uses the index
What am I missing?
It is avoiding a double lookup
Your index is
(external_id, external_type, type), but in order to get all the information necessary for the query it would have to use that index to find the rows, then use the
id that is automatically included at the end of that index to look up the
updated_at columns from the main table.
The optimizer makes the judgement that it would just be simpler to go straight to the main table to begin with, and so ignores the index.
You can see evidence of this fact with your statement:
If I change the first line of the SELECT statement to remove users.created_at, users.updated_at, it uses the index
Once you remove those columns, it no longer has to do a double lookup to complete the query. The single lookup from the index is what gets it to choose to use that index.
As for the following:
If I change the query’s WHERE clause to add and users.type="Blah", it uses the index
I would guess that the optimizer now thinks the double lookup is worth it, if it can reduce the rows enough with this more selective query. Understanding the reasoning of the optimizer is not always easy, but this seems like the most obvious reason.
To get it to use the index, you just need to make it so it doesn't need to perform a double lookup by making it a covering index.
(external_id, external_type, type, created_at, updated_at)
This index will allow it to avoid the double lookup, as it can filter on the first columns, and then just use the remaining columns in the index to satisfy the SELECT for that table without having to jump back to the main table.
Why is my SQL query not using the table's composite index?, It is avoiding a double lookup. Your index is (external_id, external_type, type) , but in order to get all the information necessary for the query it In your case, the query optimizer may think the table scan is the fastest if the statistics suggest most of the data in the table users with external_id = 3 and external_type = 'Owner' because no index on the table covers the columns being selected, and the query engine needs to do lookups for the data based on the index if index is used.
MySQL "NOT IN" query, To use IN, you must have a set, use this syntax instead: SELECT * FROM Table1 WHERE Table1.principal NOT IN (SELECT principal FROM table2). MYSQL Query for finding rows without certain associated rows in another table. Tag: mysql I am sure the answer is out there somewhere, but I am having a hard time articulating what I need, so I figured it best to give an example.
Not sure what version of mysql you are using. Before 8.0, mysql innodb does not persist the statistics, and the statistics in memory can hardly represent the data if your data is skewed. In your case, the query optimizer may think the table scan is the fastest if the statistics suggest most of the data in the table users with external_id = 3 and external_type = 'Owner' because no index on the table covers the columns being selected, and the query engine needs to do lookups for the data based on the index if index is used.
When you change to SELECT the only columns from the index, the index becomes the covering index and the query engine will not need to do the lookup.
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- Your query and your table definitions are inconsistent. The query has the external fields coming from
- @GordonLinoff Sorry, I updated it!
- What version of MySQL?
- Please provide
SHOW CREATE TABLE; there could be collation or other issues.
- This is a great answer - thank you for explaining so thoroughly! :)
- Sorry, I got the query wrong in the description trying to anonymize some company data -- updated it!!
- And when you add users.type="Blah", the index becomes more selective and the optimizer decides to use this index. If your query can definitely benefit from the index, you add a query hint to force mysql to use the index