Why Your MySQL Index Might Not Work: 10 Common Pitfalls and How to Detect Them

Many developers can talk about the advantages and disadvantages of creating indexes, but sometimes an index does not take effect even when it is defined. This article shows how to determine whether a MySQL index is effective and lists common scenarios that cause index failure.

How to Determine If a Database Index Is Effective

First, use EXPLAIN to see how MySQL utilizes indexes for a SELECT statement. EXPLAIN displays how MySQL processes the query and which indexes are used, helping you choose better indexes and write more optimized queries.

Example: create an index on the name column of a user table.

Run EXPLAIN on a query:

The EXPLAIN output includes several columns, whose meanings are:

table : indicates which table the row refers to.

type : the join type, from best to worst: const, eq_ref, ref, range, index, ALL.

possible_keys : indexes that could be used for this table; empty means none.

key : the index actually used; NULL means no index was used. In rare cases MySQL may choose a suboptimal index, which can be forced with USE INDEX or ignored with IGNORE INDEX.

key_len : length of the used index; shorter is better without losing precision.

ref : shows which column or constant the index is compared to.

rows : estimated number of rows MySQL must examine to return the result.

Extra : additional information about how MySQL resolves the query.

For detailed explanations of each column, refer to the MySQL official documentation.

Scenarios That Cause Index Ineffectiveness

Avoid using != or <> in WHERE clauses; they cause a full table scan.

Avoid using OR to combine conditions; it can prevent index usage even if some conditions are indexed.

For composite indexes, the leftmost prefix must be used; otherwise the index is ignored.

If the column type is a string, the value must be quoted in the condition; otherwise the index is not used.

LIKE patterns that start with % disable index usage.

Avoid applying expressions to indexed columns in WHERE clauses; this leads to a full scan.

Avoid applying functions to indexed columns; this also disables the index.

Do not place functions or arithmetic on the left side of = in WHERE clauses; the index may not be used.

If MySQL estimates that a full table scan is cheaper than using the index, it will skip the index.

Indexes on low‑cardinality columns (many duplicate values) are often ineffective; for example, a column with only five distinct values in a large table may cause the optimizer to prefer a full scan.

These ten points cover common reasons why an index may become ineffective; keep them in mind during development and interviews.