8.8.2 EXPLAIN Output Format

id (JSON name: select_id)

The SELECT identifier. This is the sequential number of the SELECT within the query. The value can be NULL if the row refers to the union result of other rows. In this case, the table column shows a value like <unionM,N> to indicate that the row refers to the union of the rows with id values of M and N.

select_type (JSON name: none)

The type of SELECT, which can be any of those shown in the following table. A JSON-formatted EXPLAIN exposes the SELECT type as a property of a query_block, unless it is SIMPLE or PRIMARY. The JSON names (where applicable) are also shown in the table.

DEPENDENT typically signifies the use of a correlated subquery. See Section 13.2.11.7, “Correlated Subqueries”.

DEPENDENT SUBQUERY evaluation differs from UNCACHEABLE SUBQUERY evaluation. For DEPENDENT SUBQUERY, the subquery is re-evaluated only once for each set of different values of the variables from its outer context. For UNCACHEABLE SUBQUERY, the subquery is re-evaluated for each row of the outer context.

When you specify FORMAT=JSON with EXPLAIN, the output has no single property directly equivalent to select_type; the query_block property corresponds to a given SELECT. Properties equivalent to most of the SELECT subquery types just shown are available (an example being materialized_from_subquery for MATERIALIZED), and are displayed when appropriate. There are no JSON equivalents for SIMPLE or PRIMARY.

The select_type value for non-SELECT statements displays the statement type for affected tables. For example, select_type is DELETE for DELETE statements.

table (JSON name: table_name)

The name of the table to which the row of output refers. This can also be one of the following values:

partitions (JSON name: partitions)

The partitions from which records would be matched by the query. The value is NULL for nonpartitioned tables. See Section 23.3.5, “Obtaining Information About Partitions”.

type (JSON name: access_type)

The join type. For descriptions of the different types, see EXPLAIN Join Types.

possible_keys (JSON name: possible_keys)

The possible_keys column indicates the indexes from which MySQL can choose to find the rows in this table. Note that this column is totally independent of the order of the tables as displayed in the output from EXPLAIN. That means that some of the keys in possible_keys might not be usable in practice with the generated table order.

If this column is NULL (or undefined in JSON-formatted output), there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to check whether it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. See Section 13.1.9, “ALTER TABLE Syntax”.

To see what indexes a table has, use SHOW INDEX FROM tbl_name.

key (JSON name: key)

The key column indicates the key (index) that MySQL actually decided to use. If MySQL decides to use one of the possible_keys indexes to look up rows, that index is listed as the key value.

It is possible that key will name an index that is not present in the possible_keys value. This can happen if none of the possible_keys indexes are suitable for looking up rows, but all the columns selected by the query are columns of some other index. That is, the named index covers the selected columns, so although it is not used to determine which rows to retrieve, an index scan is more efficient than a data row scan.

For InnoDB, a secondary index might cover the selected columns even if the query also selects the primary key because InnoDB stores the primary key value with each secondary index. If key is NULL, MySQL found no index to use for executing the query more efficiently.

To force MySQL to use or ignore an index listed in the possible_keys column, use FORCE INDEX, USE INDEX, or IGNORE INDEX in your query. See Section 8.9.4, “Index Hints”.

For MyISAM tables, running ANALYZE TABLE helps the optimizer choose better indexes. For MyISAM tables, myisamchk --analyze does the same. See Section 13.7.3.1, “ANALYZE TABLE Syntax”, and Section 7.6, “MyISAM Table Maintenance and Crash Recovery”.

key_len (JSON name: key_length)

The key_len column indicates the length of the key that MySQL decided to use. The value of key_len enables you to determine how many parts of a multiple-part key MySQL actually uses. If the key column says NULL, the len_len column also says NULL.

Due to the key storage format, the key length is one greater for a column that can be NULL than for a NOT NULL column.

ref (JSON name: ref)

The ref column shows which columns or constants are compared to the index named in the key column to select rows from the table.

If the value is func, the value used is the result of some function. To see which function, use SHOW WARNINGS following EXPLAIN to see the extended EXPLAIN output. The function might actually be an operator such as an arithmetic operator.

rows (JSON name: rows)

The rows column indicates the number of rows MySQL believes it must examine to execute the query.

For InnoDB tables, this number is an estimate, and may not always be exact.

filtered (JSON name: filtered)

The filtered column indicates an estimated percentage of table rows that will be filtered by the table condition. The maximum value is 100, which means no filtering of rows occurred. Values decreasing from 100 indicate increasing amounts of filtering. rows shows the estimated number of rows examined and rows × filtered shows the number of rows that will be joined with the following table. For example, if rows is 1000 and filtered is 50.00 (50%), the number of rows to be joined with the following table is 1000 × 50% = 500.

Extra (JSON name: none)

This column contains additional information about how MySQL resolves the query. For descriptions of the different values, see EXPLAIN Extra Information.

There is no single JSON property corresponding to the Extra column; however, values that can occur in this column are exposed as JSON properties, or as the text of the message property.

system

The table has only one row (= system table). This is a special case of the const join type.

const

The table has at most one matching row, which is read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast because they are read only once.

const is used when you compare all parts of a PRIMARY KEY or UNIQUE index to constant values. In the following queries, tbl_name can be used as a const table:

eq_ref

One row is read from this table for each combination of rows from the previous tables. Other than the system and const types, this is the best possible join type. It is used when all parts of an index are used by the join and the index is a PRIMARY KEY or UNIQUE NOT NULL index.

eq_ref can be used for indexed columns that are compared using the = operator. The comparison value can be a constant or an expression that uses columns from tables that are read before this table. In the following examples, MySQL can use an eq_ref join to process ref_table:

ref

All rows with matching index values are read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this is a good join type.

ref can be used for indexed columns that are compared using the = or <=> operator. In the following examples, MySQL can use a ref join to process ref_table:

fulltext

The join is performed using a FULLTEXT index.

ref_or_null

This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. This join type optimization is used most often in resolving subqueries. In the following examples, MySQL can use a ref_or_null join to process ref_table:

See Section 8.2.1.13, “IS NULL Optimization”.

index_merge

This join type indicates that the Index Merge optimization is used. In this case, the key column in the output row contains a list of indexes used, and key_len contains a list of the longest key parts for the indexes used. For more information, see Section 8.2.1.3, “Index Merge Optimization”.

unique_subquery

This type replaces eq_ref for some IN subqueries of the following form:

unique_subquery is just an index lookup function that replaces the subquery completely for better efficiency.

index_subquery

This join type is similar to unique_subquery. It replaces IN subqueries, but it works for nonunique indexes in subqueries of the following form:

range

Only rows that are in a given range are retrieved, using an index to select the rows. The key column in the output row indicates which index is used. The key_len contains the longest key part that was used. The ref column is NULL for this type.

range can be used when a key column is compared to a constant using any of the =, <>, >, >=, <, <=, IS NULL, <=>, BETWEEN, LIKE, or IN() operators:

index

The index join type is the same as ALL, except that the index tree is scanned. This occurs two ways:

MySQL can use this join type when the query uses only columns that are part of a single index.

ALL

A full table scan is done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. Normally, you can avoid ALL by adding indexes that enable row retrieval from the table based on constant values or column values from earlier tables.

Child of 'table' pushed join@1 (JSON: message text)

This table is referenced as the child of table in a join that can be pushed down to the NDB kernel. Applies only in NDB Cluster, when pushed-down joins are enabled. See the description of the ndb_join_pushdown server system variable for more information and examples.

const row not found (JSON property: const_row_not_found)

For a query such as SELECT ... FROM tbl_name, the table was empty.

Deleting all rows (JSON property: message)

For DELETE, some storage engines (such as MyISAM) support a handler method that removes all table rows in a simple and fast way. This Extra value is displayed if the engine uses this optimization.

Distinct (JSON property: distinct)

MySQL is looking for distinct values, so it stops searching for more rows for the current row combination after it has found the first matching row.

FirstMatch(tbl_name) (JSON property: first_match)

The semi-join FirstMatch join shortcutting strategy is used for tbl_name.

Full scan on NULL key (JSON property: message)

This occurs for subquery optimization as a fallback strategy when the optimizer cannot use an index-lookup access method.

Impossible HAVING (JSON property: message)

The HAVING clause is always false and cannot select any rows.

Impossible WHERE (JSON property: message)

The WHERE clause is always false and cannot select any rows.

Impossible WHERE noticed after reading const tables (JSON property: message)

MySQL has read all const (and system) tables and notice that the WHERE clause is always false.

LooseScan(m..n) (JSON property: message)

The semi-join LooseScan strategy is used. m and n are key part numbers.

No matching min/max row (JSON property: message)

No row satisfies the condition for a query such as SELECT MIN(...) FROM ... WHERE condition.

no matching row in const table (JSON property: message)

For a query with a join, there was an empty table or a table with no rows satisfying a unique index condition.

No matching rows after partition pruning (JSON property: message)

For DELETE or UPDATE, the optimizer found nothing to delete or update after partition pruning. It is similar in meaning to Impossible WHERE for SELECT statements.

No tables used (JSON property: message)

The query has no FROM clause, or has a FROM DUAL clause.

For INSERT or REPLACE statements, EXPLAIN displays this value when there is no SELECT part. For example, it appears for EXPLAIN INSERT INTO t VALUES(10) because that is equivalent to EXPLAIN INSERT INTO t SELECT 10 FROM DUAL.

Not exists (JSON property: message)

MySQL was able to do a LEFT JOIN optimization on the query and does not examine more rows in this table for the previous row combination after it finds one row that matches the LEFT JOIN criteria. Here is an example of the type of query that can be optimized this way:

Assume that t2.id is defined as NOT NULL. In this case, MySQL scans t1 and looks up the rows in t2 using the values of t1.id. If MySQL finds a matching row in t2, it knows that t2.id can never be NULL, and does not scan through the rest of the rows in t2 that have the same id value. In other words, for each row in t1, MySQL needs to do only a single lookup in t2, regardless of how many rows actually match in t2.

Plan isn't ready yet (JSON property: none)

This value occurs with EXPLAIN FOR CONNECTION when the optimizer has not finished creating the execution plan for the statement executing in the named connection. If execution plan output comprises multiple lines, any or all of them could have this Extra value, depending on the progress of the optimizer in determining the full execution plan.

Range checked for each record (index map: N) (JSON property: message)

MySQL found no good index to use, but found that some of indexes might be used after column values from preceding tables are known. For each row combination in the preceding tables, MySQL checks whether it is possible to use a range or index_merge access method to retrieve rows. This is not very fast, but is faster than performing a join with no index at all. The applicability criteria are as described in Section 8.2.1.2, “Range Optimization”, and Section 8.2.1.3, “Index Merge Optimization”, with the exception that all column values for the preceding table are known and considered to be constants.

Indexes are numbered beginning with 1, in the same order as shown by SHOW INDEX for the table. The index map value N is a bitmask value that indicates which indexes are candidates. For example, a value of 0x19 (binary 11001) means that indexes 1, 4, and 5 will be considered.

Recursive (JSON property: recursive)

This indicates that the row applies to the recursive SELECT part of a recursive common table expression. See Section 13.2.13, “WITH Syntax (Common Table Expressions)”.

Rematerialize (JSON property: rematerialize)

Rematerialize (X,...) is displayed in the EXPLAIN row for table T, where X is any lateral derived table whose rematerialization is triggered when a new row of T is read. For example:

The content of the derived table is rematerialized to bring it up to date each time a new row of t is processed by the top query.

Scanned N databases (JSON property: message)

This indicates how many directory scans the server performs when processing a query for INFORMATION_SCHEMA tables, as described in Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”. The value of N can be 0, 1, or all.

Select tables optimized away (JSON property: message)

The optimizer determined 1) that at most one row should be returned, and 2) that to produce this row, a deterministic set of rows must be read. When the rows to be read can be read during the optimization phase (for example, by reading index rows), there is no need to read any tables during query execution.

The first condition is fulfilled when the query is implicitly grouped (contains an aggregate function but no GROUP BY clause). The second condition is fulfilled when one row lookup is performed per index used. The number of indexes read determines the number of rows to read.

Consider the following implicitly grouped query:

Suppose that MIN(c1) can be retrieved by reading one index row and MIN(c2) can be retrieved by reading one row from a different index. That is, for each column c1 and c2, there exists an index where the column is the first column of the index. In this case, one row is returned, produced by reading two deterministic rows.

This Extra value does not occur if the rows to read are not deterministic. Consider this query:

Suppose that (c1, c2) is a covering index. Using this index, all rows with c1 <= 10 must be scanned to find the minimum c2 value. By contrast, consider this query:

In this case, the first index row with c1 = 10 contains the minimum c2 value. Only one row must be read to produce the returned row.

For storage engines that maintain an exact row count per table (such as MyISAM, but not InnoDB), this Extra value can occur for COUNT(*) queries for which the WHERE clause is missing or always true and there is no GROUP BY clause. (This is an instance of an implicitly grouped query where the storage engine influences whether a deterministic number of rows can be read.)

Skip_open_table, Open_frm_only, Open_full_table (JSON property: message)

These values indicate file-opening optimizations that apply to queries for INFORMATION_SCHEMA tables.

Start temporary, End temporary (JSON property: message)

This indicates temporary table use for the semi-join Duplicate Weedout strategy.

unique row not found (JSON property: message)

For a query such as SELECT ... FROM tbl_name, no rows satisfy the condition for a UNIQUE index or PRIMARY KEY on the table.

Using filesort (JSON property: using_filesort)

MySQL must do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the join type and storing the sort key and pointer to the row for all rows that match the WHERE clause. The keys then are sorted and the rows are retrieved in sorted order. See Section 8.2.1.14, “ORDER BY Optimization”.

Using index (JSON property: using_index)

The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This strategy can be used when the query uses only columns that are part of a single index.

For InnoDB tables that have a user-defined clustered index, that index can be used even when Using index is absent from the Extra column. This is the case if type is index and key is PRIMARY.

Using index condition (JSON property: using_index_condition)

Tables are read by accessing index tuples and testing them first to determine whether to read full table rows. In this way, index information is used to defer (“push down”) reading full table rows unless it is necessary. See Section 8.2.1.5, “Index Condition Pushdown Optimization”.

Using index for group-by (JSON property: using_index_for_group_by)

Similar to the Using index table access method, Using index for group-by indicates that MySQL found an index that can be used to retrieve all columns of a GROUP BY or DISTINCT query without any extra disk access to the actual table. Additionally, the index is used in the most efficient way so that for each group, only a few index entries are read. For details, see Section 8.2.1.15, “GROUP BY Optimization”.

Using index for skip scan (JSON property: using_index_for_skip_scan)

Indicates that the Skip Scan access method is used. See Skip Scan Range Access Method.

Using join buffer (Block Nested Loop), Using join buffer (Batched Key Access) (JSON property: using_join_buffer)

Tables from earlier joins are read in portions into the join buffer, and then their rows are used from the buffer to perform the join with the current table. (Block Nested Loop) indicates use of the Block Nested-Loop algorithm and (Batched Key Access) indicates use of the Batched Key Access algorithm. That is, the keys from the table on the preceding line of the EXPLAIN output will be buffered, and the matching rows will be fetched in batches from the table represented by the line in which Using join buffer appears.

In JSON-formatted output, the value of using_join_buffer is always either one of Block Nested Loop or Batched Key Access.

Using MRR (JSON property: message)

Tables are read using the Multi-Range Read optimization strategy. See Section 8.2.1.10, “Multi-Range Read Optimization”.

Using sort_union(...), Using union(...), Using intersect(...) (JSON property: message)

These indicate the particular algorithm showing how index scans are merged for the index_merge join type. See Section 8.2.1.3, “Index Merge Optimization”.

Using temporary (JSON property: using_temporary_table)

To resolve the query, MySQL needs to create a temporary table to hold the result. This typically happens if the query contains GROUP BY and ORDER BY clauses that list columns differently.

Using where (JSON property: attached_condition)

A WHERE clause is used to restrict which rows to match against the next table or send to the client. Unless you specifically intend to fetch or examine all rows from the table, you may have something wrong in your query if the Extra value is not Using where and the table join type is ALL or index.

Using where has no direct counterpart in JSON-formatted output; the attached_condition property contains any WHERE condition used.

Using where with pushed condition (JSON property: message)

This item applies to NDB tables only. It means that NDB Cluster is using the Condition Pushdown optimization to improve the efficiency of a direct comparison between a nonindexed column and a constant. In such cases, the condition is “pushed down” to the cluster's data nodes and is evaluated on all data nodes simultaneously. This eliminates the need to send nonmatching rows over the network, and can speed up such queries by a factor of 5 to 10 times over cases where Condition Pushdown could be but is not used. For more information, see Section 8.2.1.4, “Engine Condition Pushdown Optimization”.

Zero limit (JSON property: message)

The query had a LIMIT 0 clause and cannot select any rows.

The columns being compared have been declared as follows.

The tables have the following indexes.

The tt.ActualPC values are not evenly distributed.