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2011-07-30MySQL 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.
Performs a B-tree traversal only. 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 also “Anatomy of an SQL Index”.
- ref, range
Performs a B-tree traversal and walks through the leaf nodes to find all matching entries (similar to
INDEX RANGE SCAN). See also “Anatomy of an SQL Index”.
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”. 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”.