SQL - Indexes (2024)

FAQs

Is it bad to have too many indexes in SQL? ›

The downside of excessive indexing: Overhead: Each additional index consumes storage space and incurs maintenance costs during data modification operations (e.g., INSERT, UPDATE, DELETE). This can impact overall system performance.

Indexes should not be used on tables containing few records. Tables that have frequent, large batch updates or insert operations. Indexes should not be used on columns that contain a high number of NULL values. Indexes should not be used on the columns that are frequently manipulated.

It is important to specify the correct order of the columns in indexes. As a rule of thumb, the columns that are used for decision making in WHERE clauses, and conditions such as greater than (>), less than (<) etc, should be placed before the columns not involved in these clauses.

There are a few reasons why it is not a good idea to index all columns. First, the INSERT, UPDATE, and DELETE operations will take more time. Second, this kind of index will take up more disk space.

Every index takes up additsional storage, can slow down write operations, and can complicate the query optimizer's job, so they aren't always guaranteed to improve performance. Ultimately the decision to add indexes should be based on the specific needs of your application and the trade-offs you are willing to make.

We should also be careful to not make an index for each query as creating indexes also take storage and when the amount of data is huge it will create a problem. Therefore, it's important to carefully consider which columns to index based on the needs of your application.

There is not an "optimal" number of indexes for a table. The number of indexes completely depends on how you query the table. Having unused indexes wastes disks space and resources to maintain the index. As well as user time during insert/update/deletes.

Multiple indexes can improve the performance of queries that use different combinations of columns. A single index can be used to optimize queries that use multiple columns. Each index is stored separately and the size of the index can grow as needed.

No, you should not index all of your columns, and there's several reasons for this: There is a cost to maintain each index during an insert, update or delete statement, that will cause each of those transactions to take longer. It will increase the storage required since each index takes up space on disk.

What makes a good index SQL? ›

You should build an index based on the predicates in the Where clause. For example, you can consider columns used in the Where clause, SQL joins, like, order by, group by predicates, and so on. You should join tables in a way that reduces the number of rows for the rest of the query.

Indexes should not be used on small tables. Indexes should not be used on columns that return a high percentage of data rows when used as a filter condition in a query's WHERE clause. For instance, you would not have an entry for the word "the" or "and" in the index of a book.

The database may mark an index unusable in various situations, including when an index creation or rebuild fails midway. For example, when the table data becomes more up-to-date than the indexes on that table, SQL*Loader leaves the index in an unusable state.

Indexing small tables may not be optimal because it can take the query optimizer longer to traverse the index searching for data than to perform a simple table scan. Therefore, indexes on small tables might never be used, but must still be maintained as data in the table changes.

Creating unnecessary indexes leads to a bloated collection and slow writes. Consider each query your application performs and whether it justifies an index. Remove indexes that are unused, either because the field is not used to query the database or because the index is redundant.

A lot of indexes make INSERT, DELETE, and UPDATE slower. Many indexes increase the number of “choices” that the query optimizer has to consider for possible query plans. This may make accesses to the table generally slower, particularly for complex joins.

Each table can have up to 999 nonclustered indexes, regardless of how the indexes are created: either implicitly with PRIMARY KEY and UNIQUE constraints, or explicitly with CREATE INDEX . For indexed views, nonclustered indexes can be created only on a view that has a unique clustered index already defined.