#redundant #sql #server
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How to Find Redundant Indexes in SQL
The following two indexes are redundant in most SQL databases:
It is usually safe to drop the first index, because all queries that query the LAST_NAME column only can still profit from the second index I_ACTOR_2. The reason being that LAST_NAME is the first column of the composite index I_ACTOR_2 (it would be a different story, if it weren t the first column).
Note: It is usually safe to drop the first index, because the benefits probably outweigh the cost:
Benefits of dropping
- Querying a composite index can be slightly slower as can be seen in the below benchmark
Let s see the costs of dropping the index below for Oracle, PostgreSQL, and SQL Server in this particular case (beware as always when interpreting benchmarks, they heavily depend on a lot of context, especially data size!)
The result being:
The fastest query execution in the above result yields 1, the other executions are multiples of 1. Yes, there s a 10% difference in this case, so as you can see. The benefits (faster insertions) certainly should outweight the cost (slower queries), so, don t apply this advice in a read-heavy / write-rarely database.
A similar difference can be seen in a PostgreSQL benchmark. No hints can be used to choose indexes, so we re simply creating two tables:
As can be seen, predictably, in all databases the smaller non-composite index is slightly faster for this type of query than the composite index. In this particular benchmark, this is specifically true because the composite index acts as a covering index.
Yet both indexes can be used for the query in a reasonable way, so if disk space / insertion speed is an issue, the redundant single-column index can be dropped.
How to find such indexes
The following query will help you detect such indexes in Oracle, PostgreSQL, and SQL Server:
In short, it lists all the indexes whose columns are a prefix of another index s columns
Get ready for a really nifty query. Here s how to discover redundant indexes in PostgreSQL, which unfortunately doesn t seem to have an easy, out-of-the-box dictionary view to discover index columns:
Now, SQL Server doesn t have a nice STRING_AGG function (yet ), but we can work around this using STUFF and XML to get the same query.
Of course, there are other solutions using recursive SQL, but I m too lazy to translate the simple string pattern-matching approach to something recursive.
Now go run the above query on your production database and Very carefully and reasonably think about whether you really want to drop those indexes