What is an index (database)?
A database index is a lookup structure that lets the database find rows fast instead of reading the whole table.
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An index is an extra data structure the database keeps next to a table so it can jump straight to matching rows. The index at the back of a textbook is exactly this: instead of reading all 400 pages to find the word photosynthesis, you look it up and go to page 212. Without an index the database must scan every row, which is fine for 100 rows and painful for 100 million.
You add indexes on columns you search or join by often, such as an email column or a foreign key. Every index speeds up reads but slows down writes a little and takes disk space, so you do not index everything. EXPLAIN shows whether a slow query is using an index or scanning the table.
CREATE INDEX idx_customers_email ON customers (email);
EXPLAIN SELECT * FROM customers WHERE email = '[email protected]';See also: What is SQL? and What is a primary key?
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See also
- 0070 Indexes and EXPLAIN: making SQL queries fast An index is a sorted lookup structure that lets the database jump to matching rows instead of reading the whole table. EXPLAIN… Advanced · Explains an idea · Any system Article Indexes & performance 15 Oct 2026
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