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Databases & SQL vocabulary · Indexes and Query Performance

Index

Meaning in databases & SQL, with examples

/ˈɪndɛks/nounCore term

In databases & SQL, index means: An extra data structure built on one or more columns that lets the database find rows quickly without reading the whole table, like the index at the back of a book.

In Turkish: indeks, dizin

How it is used

The first thing people check when a query is slow. You create, add, drop and rebuild indexes; a query 'uses' or 'hits' an index. Indexes speed up reads but slow down writes and take disk space, so adding one is always a trade-off. The default type in most databases is a B-tree.

Example sentences

  1. After we added an index on created_at, the query went from six seconds to forty milliseconds.

  2. Too many indexes on this table are slowing down our inserts.

Common collocations

  • create an index
  • add an index
  • drop an index
  • use an index
  • composite index
  • clustered indexAn index that decides the physical order in which a table's rows are stored, so a table can have only one; the other indexes are non-clustered.
  • full table scanReading every row of a table to find the ones a query needs, instead of jumping straight to them through an index.
  • execution planThe step-by-step strategy the database chooses to run a query, showing which indexes it uses, how tables are joined and how many rows it expects.
  • query tuningThe work of making slow queries faster by studying their execution plans and then adding indexes, rewriting SQL or changing database settings.
  • unique constraintA rule that stops two rows from having the same value in a column or combination of columns, for example two users with the same email address.

This word in other fields

The same English word can have a different meaning in another field:

More terms: Indexes and Query Performance

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