I keep reading about indexing, but I am confused about when dealing with multi-column filtering. Should I create multiple single indexes or a single composite index for queries that use multiple conditions in the WHERE clause? How does index order matter here?
3 answers
For multi-column filtering, a single composite index is significantly faster than multiple single-column indexes because MySQL can usually only use one index per table access path. When designing a composite index, follow the Leftmost Prefix Rule. Place the most selective column—the one that filters out the most rows—first in the index sequence, followed by columns used in equality checks, and finally columns used for range conditions like > or <. If you change this order, the optimizer might completely ignore the index.
Does your query utilize OR conditions across those columns, or are they strictly tied together using AND operators within your application logic?
Always use covering indexes where possible. If your index contains all the columns requested in the SELECT clause, MySQL won't need to look up data in the actual table.
Covering indexes are a golden rule. It completely bypasses data page lookups, making select operations execute almost instantly even on tables with millions of rows.
Patrick, if they use OR conditions, a composite index will actually fail to optimize the query. In that specific scenario, you would either need separate indexes combined via index_merge or rewrite the query using UNION to make it run faster.