The EXPLAIN statement returns CockroachDB’s query plan to execute DELETE, INSERT, SELECT or UPDATE statements. You can then use this information to optimize those queries.

Query Optimization

Using EXPLAIN’s output, you can optimize your queries by taking the following points into consideration:

  • Queries with fewer levels execute more quickly. Restructuring queries to require fewer levels of processing will generally improve performance.

  • Avoid scanning an entire table, which is the slowest way to access data. You can avoid this by creating indexes that contain at least one of the columns that the query is filtering in its WHERE clause.

    You can find out if your queries are performing entire table scans by using EXPLAIN to see which:

    • Indexes the query uses; shown as the Description value of rows with the Field value of table

    • Key values in the index are being scanned; shown as the Description value of rows with the Field value of spans

    For more information, see Find the Indexes and Key Ranges a Query Uses.

Synopsis

EXPLAIN ( EXPRS METADATA QUALIFY VERBOSE TYPES , ) explainable_stmt

Required Privileges

The user requires the appropriate privileges for the statement being explained.

Parameters

Parameter Description
EXPRS Include the SQL expressions that are involved in each processing stage.
QUALIFY Include table names when referencing columns, which might be important to verify the behavior of joins across tables with the same column names.

To list qualified names, QUALIFY requires you to include the EXPRS option.
METADATA Include the columns each level uses in the Columns column, as well as Ordering detail.
VERBOSE Imply the EXPRS, METADATA, and QUALIFY options.
TYPES Include the intermediate data types CockroachDB chooses to evaluate intermediate SQL expressions.

TYPES also implies METADATA and EXPRS options.
explainable_stmt The DELETE, INSERT, SELECT or UPDATE statement you want detail about.
EXPLAIN also includes other modes besides query plans that are useful only to CockroachDB developers, which are not documented here.

Success Responses

Successful EXPLAIN statements return tables with the following columns:

Column Description
Level The level of hierarchy of the query plan.

0 represents the last processing stage that produces the results sent to the SQL client receiving the data; the highest level represents the operation at the key-value layer accessing data.

The query plan has a tree structure; it is thus possible to see multiple processing stages at the same level, which indicates they are sibling stages feeding data to the previous processing stage with a lower level.
Type The query plan node’s type, which are described in the CockroachDB source on GitHub.
Field The type of parameter being used by the query plan node.
Description Additional information describing the Field value.
Columns The columns provided to the processes at lower levels of the hierarchy.

This column displays only if the METADATA option is specified or implied.
Ordering The order in which results are presented to the processes at lower levels of the hierarchy.

This column displays only if the METADATA option is specified or implied.

Examples

Default Query Plans

By default, EXPLAIN includes the least detail about the query plan but can be useful to find out which indexes and keys a query uses.

> EXPLAIN SELECT * FROM kv WHERE v > 3 ORDER BY v;
+-------+------+-------+-------------+
| Level | Type | Field | Description |
+-------+------+-------+-------------+
|     0 | sort |       |             |
|     0 |      | order | +v          |
|     1 | scan |       |             |
|     1 |      | table | [email protected]  |
|     1 |      | spans | ALL         |
+-------+------+-------+-------------+

EXPRS Option

The EXPRS option includes SQL expressions that are involved in each processing stage, providing more granular detail about which portion of your query is represented at each level.

> EXPLAIN (EXPRS) SELECT * FROM kv WHERE v > 3 ORDER BY v;
+-------+------+--------+-------------+
| Level | Type | Field  | Description |
+-------+------+--------+-------------+
|     0 | sort |        |             |
|     0 |      | order  | +v          |
|     1 | scan |        |             |
|     1 |      | table  | [email protected]  |
|     1 |      | spans  | ALL         |
|     1 |      | filter | v > 3       |
+-------+------+--------+-------------+

METADATA Option

The METADATA option includes detail about which columns are being used by each level, as well as how columns are being ordered.

> EXPLAIN (METADATA) SELECT * FROM kv WHERE v > 3 ORDER BY v;
+-------+------+-------+-------------+---------+--------------+
| Level | Type | Field | Description | Columns |   Ordering   |
+-------+------+-------+-------------+---------+--------------+
|     0 | sort |       |             | (k, v)  | +v           |
|     0 |      | order | +v          |         |              |
|     1 | scan |       |             | (k, v)  | +k,+v,unique |
|     1 |      | table | [email protected]  |         |              |
|     1 |      | spans | ALL         |         |              |
+-------+------+-------+-------------+---------+--------------+

When looking at the Ordering column, we can also sort by descending (DESC) values of k, which is indicated by the - sign.

> EXPLAIN (METADATA) SELECT * FROM kv WHERE v > 3 ORDER BY v DESC;
+-------+------+-------+-------------+---------+--------------+
| Level | Type | Field | Description | Columns |   Ordering   |
+-------+------+-------+-------------+---------+--------------+
|     0 | sort |       |             | (k, v)  | -v           |
|     0 |      | order | -v          |         |              |
|     1 | scan |       |             | (k, v)  | +k,+v,unique |
|     1 |      | table | [email protected]  |         |              |
|     1 |      | spans | ALL         |         |              |
+-------+------+-------+-------------+---------+--------------+
In some cases the Ordering details report a column ordering with an equal sign (e.g. =k). This is a side effect of the internal ordering analysis performed by CockroachDB and merely indicates that CockroachDB has found that only one row matches a WHERE expression.

QUALIFY Option

QUALIFY uses <table name>.<column name> notation for columns in the query plan. However, QUALIFY must be used with EXPRS to show the SQL values used.

> EXPLAIN (EXPRS, QUALIFY) SELECT a.v, b.v FROM t.kv AS a, t.kv AS b;
+-------+--------+----------+-------------+
| Level |  Type  |  Field   | Description |
+-------+--------+----------+-------------+
|     0 | render |          |             |
|     0 |        | render 0 | a.v         |
|     0 |        | render 1 | b.v         |
|     1 | join   |          |             |
|     1 |        | type     | cross       |
|     2 | scan   |          |             |
|     2 |        | table    | [email protected]  |
|     2 | scan   |          |             |
|     2 |        | table    | [email protected]  |
+-------+--------+----------+-------------+

You can contrast this with the same statement not including the QUALIFY option to see that the column references are not qualified, which can lead to ambiguity if multiple tables have columns with the same names.

>  EXPLAIN (EXPRS) SELECT a.v, b.v FROM t.kv AS a, t.kv AS b;
+-------+--------+----------+-------------+
| Level |  Type  |  Field   | Description |
+-------+--------+----------+-------------+
|     0 | render |          |             |
|     0 |        | render 0 | v           |
|     0 |        | render 1 | v           |
|     1 | join   |          |             |
|     1 |        | type     | cross       |
|     2 | scan   |          |             |
|     2 |        | table    | [email protected]  |
|     2 | scan   |          |             |
|     2 |        | table    | [email protected]  |
+-------+--------+----------+-------------+

VERBOSE Option

The VERBOSE option implies the EXPRS, METADATA, and QUALIFY options.

> EXPLAIN (VERBOSE) SELECT * FROM kv AS a JOIN kv USING (k) WHERE a.v > 3 ORDER BY a.v DESC;
+-------+--------+----------+-------------+-------------------------------------------------+--------------+
| Level |  Type  |  Field   | Description |                     Columns                     |   Ordering   |
+-------+--------+----------+-------------+-------------------------------------------------+--------------+
|     0 | sort   |          |             | (k, v, v)                                       | -v           |
|     0 |        | order    | -v          |                                                 |              |
|     1 | render |          |             | (k, v, v)                                       |              |
|     1 |        | render 0 | k           |                                                 |              |
|     1 |        | render 1 | a.v         |                                                 |              |
|     1 |        | render 2 | bank.kv.v   |                                                 |              |
|     2 | join   |          |             | (k, k[hidden,omitted], v, k[hidden,omitted], v) |              |
|     2 |        | type     | inner       |                                                 |              |
|     2 |        | equality | (k) = (k)   |                                                 |              |
|     3 | scan   |          |             | (k, v)                                          | +k,+v,unique |
|     3 |        | table    | [email protected]  |                                                 |              |
|     3 |        | spans    | ALL         |                                                 |              |
|     3 |        | filter   | v > 3       |                                                 |              |
|     3 | scan   |          |             | (k, v)                                          | +k,+v,unique |
|     3 |        | table    | [email protected]  |                                                 |              |
+-------+--------+----------+-------------+-------------------------------------------------+--------------+

TYPES Option

The TYPES mode includes the types of the values used in the query plan, as well as implying the METADATA and EXPRS options.

> EXPLAIN (TYPES) SELECT * FROM kv WHERE v > 3 order by v;
+-------+------+--------+-----------------------------+----------------+--------------+
| Level | Type | Field  |         Description         |    Columns     |   Ordering   |
+-------+------+--------+-----------------------------+----------------+--------------+
|     0 | sort |        |                             | (k int, v int) | +v           |
|     0 |      | order  | +v                          |                |              |
|     1 | scan |        |                             | (k int, v int) | +k,+v,unique |
|     1 |      | table  | [email protected]                  |                |              |
|     1 |      | spans  | ALL                         |                |              |
|     1 |      | filter | ((v)[int] > (3)[int])[bool] |                |              |
+-------+------+--------+-----------------------------+----------------+--------------+

Find the Indexes and Key Ranges a Query Uses

You can use EXPLAIN to understand which indexes and key ranges queries use, which can help you ensure a query isn’t performing a full table scan.

> CREATE TABLE kv (k INT PRIMARY KEY, v INT);

Because column v is not indexed, queries filtering on it alone scan the entire table:

> EXPLAIN SELECT * FROM kv WHERE v BETWEEN 4 AND 5;
+-------+------+-------+-------------+
| Level | Type | Field | Description |
+-------+------+-------+-------------+
|     0 | scan |       |             |
|     0 |      | table | [email protected]  |
|     0 |      | spans | ALL         |
+-------+------+-------+-------------+

However, in the following query, column k is sorted in the primary index, so CockroachDB can avoid scanning the entire table:

> EXPLAIN SELECT * FROM kv WHERE k BETWEEN 4 AND 5;
+-------+------+-------+-------------+
| Level | Type | Field | Description |
+-------+------+-------+-------------+
|     0 | scan |       |             |
|     0 |      | table | [email protected]  |
|     0 |      | spans | /4-/6       |
+-------+------+-------+-------------+

See Also



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