How to Check the Query Is Using Index

How to check the query is using the indexes in mysql

Write "explain " in front of your query. The result will tell you which indexes might be used. For example:

explain select * from cars;

You can check the column "Extra" for the information.

how to check the query is using index

In SQL Management Studio, just type in the query, and hit Control-L (display query execution plan). There, you will be able to see whether any indexes are being used. A "table scan" means the index is not used. An "index scan" means the index is used.

How do I know if any index is used in a query | PostgreSQL 11?

You answered the question in the title yourself by running EXPLAIN. The query plan shows which indexes are used and how. For details see the chapter "Using EXPLAIN" in the manual.

As for why the query uses a sequential scan and no indexes: 25 million row, 992781 rows removed. You are fetching 24709900 rows, that's almost all rows.

This is never going to be fast.

This only going to use an index in special situations.

Using an index typically only makes sense for a small fraction of all rows. Else it would just add additional cost. Depending on a number of co-factors, the Postgres query planner starts to consider a btree index for around 5% of all rows or less. Related:

• Postgres not using index when index scan is much better option

Well, if your table rows are substantially wider than the three columns in your SELECT list, a (partial) covering index might help somewhat if you get index-only scans out of it. Again, needs to meet some preconditions. And every index also adds storage and maintenance costs.

Aside: A comment claimed NULL values couldn't be indexed. This is incorrect, NULL values can be indexed. Not as efficient as other values, but doesn't make much of a difference. Also irrelevant to the case at hand.

how to check the query is using index

In SQL Management Studio, just type in the query, and hit Control-L (display query execution plan). There, you will be able to see whether any indexes are being used. A "table scan" means the index is not used. An "index scan" means the index is used.

How to determine if an Index is required or necessary

I use Jason Strate's index analysis scripts. They tell you how much your existing indexes are used as well as how much missing indexes would have been used. I typically don't add indexes unless they make up more than 5 or 10% of the queries on a table.

Most importantly, though, it's about making sure the application responds fast enough for the users.

Jason Strate's index analysis blog articles)

These days, I use sp_BlitzIndex® when performing index analysis.

Check whether my query is using index on attribute using ANALYZE

It is not using the index, but 'parallel' does not show us that, because index scans can also be done in parallel. It is the presence of 'seq scan' and absence of '[parallel] [bitmap] index [only] scan' which tells us that. A plain EXPLAIN is enough to tell us that it doesn't use the index, but can't tell us much about why it chose not to, which I assume is what you are trying to get at.

How do I find which indexes are being used and what query is using the index?

If you have indexes in your database that are exact duplicates, delete them, period. No harm can come from removing the duplicates, but harm CAN come from the duplicates existing.

The fact that SQL Server even allows duplicate indexes to be created in the first place is ridiculous.

Here is an article on how to find unused (and missing) indexes:
http://weblogs.sqlteam.com/mladenp/archive/2009/04/08/SQL-Server---Find-missing-and-unused-indexes.aspx

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