MySQL EXPLAIN Operations
Index and Table Access
MySQL’s explain plan tends to give a false sense of safety because it says so much about indexes being used. Although technically correct, it does not mean that it is using the index efficiently. The most important information is in the
TYPE column of the MySQL’s explain output—but even there, the keyword
INDEX doesn’t indicate proper indexing.
- eq_ref, const
Performs a B-tree traversal to find one row (like
INDEX UNIQUE SCAN) and fetches additional columns from the table if needed (
TABLE ACCESS BY INDEX ROWID). The database uses this operation if a primary key or unique constraint ensures that the search criteria will match no more than one entry. See “Using Index” to check whether the table access happens or not.
- ref, range
Performs a B-tree traversal, walks through the leaf nodes to find all matching index entries (similar to
INDEX RANGE SCAN) and fetches additional columns from the primary table store if needed (
TABLE ACCESS BY INDEX ROWID). See “Using Index” to check whether the table access happens or not.
Reads the entire index—all rows—in the index order (similar to
INDEX FULL SCAN).
Reads the entire table—all rows and columns—as stored on the disk. Besides high IO rates, a table scan must also inspect all rows from the table so that it can also put a considerable load on the CPU. See also “Full Table Scan”.
- Using Index (in the “Extra” column)
When the “Extra” column shows “Using Index”, it means that the table is not accessed because the index has all the required data. Think of “using index ONLY”. However, if a clustered index is used (e.g., the
PRIMARYindex when using InnoDB) “Using Index” does not appear in the Extra column although it is technically an Index-Only Scan. See also “Clustering Data: The Second Power of Indexing”.
- PRIMARY (in the “key” or “possible_keys” column)
PRIMARYis the name of the automatically created index for the primary key.
Sorting and Grouping
- using filesort (in the “Extra” column)
“using filesort” in the Extra column indicates an explicit sort operation—no matter where the sort takes place (main memory or on disk). “Using filesort” needs large amounts of memory to materialize the intermediate result (not pipelined). See also “Indexing Order By”.
- implicit: no “using filesort” in the “Extra” column
A MySQL execution plan does not show a top-N query explicitly. If you are using the limit syntax and don’t see “using filesort” in the extra column, it is executed in a pipelined manner. See also “Querying Top-N Rows”.