The EXPLAIN statement returns CockroachDB's query plan for an explainable statement. You can then use this information to optimize the query.

Tip:

To actually execute a statement and return a physical query plan with execution statistics, use EXPLAIN ANALYZE.

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 ( VERBOSE TYPES OPT DISTSQL , ) preparable_stmt

Required privileges

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

Parameters

Parameter Description
VERBOSE Show as much information as possible about the query plan.
TYPES Include the intermediate data types CockroachDB chooses to evaluate intermediate SQL expressions.
OPT New in v2.1: Display a query plan tree if the query will be run with the cost-based optimizer. If it returns an "unsupported statement" error, the query will not be run with the cost-based optimizer and will be run with the heuristic planner.
DISTSQL New in v2.1: Generate a link to a distributed SQL physical query plan tree.
preparable_stmt The statement you want details about. All preparable statements are explainable.
Warning:

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
Tree A tree representation showing the hierarchy of the query plan.
Field The name of a parameter relevant to the query plan node immediately above.
Description Additional information for the parameter in Field.
Columns The columns provided to the processes at lower levels of the hierarchy. Included in TYPES and VERBOSE output.
Ordering The order in which results are presented to the processes at each level of the hierarchy, as well as other properties of the result set at each level. Included in TYPES and VERBOSE output.

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 index key ranges are used by a query:

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> EXPLAIN SELECT * FROM kv WHERE v > 3 ORDER BY v;
    tree    | field  | description
+-----------+--------+-------------+
  sort      |        |
   │        | order  | +v
   └── scan |        |
            | table  | kv@primary
            | spans  | ALL
            | filter | v > 3
(6 rows)

The first column shows the tree structure of the query plan; a set of properties is displayed for each node in the tree. Most importantly, for scans, you can see the index that is scanned (primary in this case) and what key ranges of the index you are scanning (in this case, a full table scan). For more information on indexes and key ranges, see the example below.

VERBOSE option

The VERBOSE 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.
  • Includes detail about which columns are being used by each level, as well as properties of the result set on that level.
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> EXPLAIN (VERBOSE) SELECT * FROM kv AS a JOIN kv USING (k) WHERE a.v > 3 ORDER BY a.v DESC;
         tree         |     field      | description |   columns    | ordering
+---------------------+----------------+-------------+--------------+----------+
  sort                |                |             | (k, v, v)    | -v
   │                  | order          | -v          |              |
   └── render         |                |             | (k, v, v)    |
        │             | render 0       | k           |              |
        │             | render 1       | v           |              |
        │             | render 2       | v           |              |
        └── join      |                |             | (k, v, k, v) |
             │        | type           | inner       |              |
             │        | equality       | (k) = (k)   |              |
             │        | mergeJoinOrder | +"(k=k)"    |              |
             ├── scan |                |             | (k, v)       | +k
             │        | table          | kv@primary  |              |
             │        | spans          | ALL         |              |
             │        | filter         | v > 3       |              |
             └── scan |                |             | (k, v)       | +k
                      | table          | kv@primary  |              |
                      | spans          | ALL         |              |
(17 rows)

TYPES option

The TYPES mode includes the types of the values used in the query plan. It also includes the SQL expressions that were involved in each processing stage, and includes the columns used by each level.

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> EXPLAIN (TYPES) SELECT * FROM kv WHERE v > 3 ORDER BY v;
    tree    | field  |         description         |    columns     | ordering
+-----------+--------+-----------------------------+----------------+----------+
  sort      |        |                             | (k int, v int) | +v
   │        | order  | +v                          |                |
   └── scan |        |                             | (k int, v int) |
            | table  | kv@primary                  |                |
            | spans  | ALL                         |                |
            | filter | ((v)[int] > (3)[int])[bool] |                |
(6 rows)

OPT option

New in v2.1: The OPT option displays a query plan tree, along with some information that was used to plan the query, if the query will be run with the cost-based optimizer. If it returns an "unsupported statement" error, the query will not be run with the cost-based optimizer and will be run with the legacy heuristic planner.

For example, the following query returns the query plan tree, which means that it will be run with the cost-based optimizer:

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> EXPLAIN(OPT) SELECT l_shipmode, avg(l_extendedprice) from lineitem GROUP BY l_shipmode;
                                     text
+-----------------------------------------------------------------------------+
group-by
├── columns: l_shipmode:15(string!null) avg:17(float)
├── grouping columns: l_shipmode:15(string!null)
├── stats: [rows=700, distinct(15)=700]
├── cost: 1207
├── key: (15)
├── fd: (15)-->(17)
├── prune: (17)
├── scan lineitem
│    ├── columns: l_extendedprice:6(float!null) l_shipmode:15(string!null)
│    ├── stats: [rows=1000, distinct(15)=700]
│    ├── cost: 1180
│    └── prune: (6,15)
└── aggregations [outer=(6)]
└── avg [type=float, outer=(6)]
└── variable: l_extendedprice [type=float, outer=(6)]
(16 rows)

In contrast, this query returns pq: unsupported statement: *tree.Insert, which means that it will use the heuristic planner instead of the cost-based optimizer:

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> EXPLAIN (OPT) INSERT INTO l_shipmode VALUES ("truck");
pq: unsupported statement: *tree.Insert

DISTSQL option

New in v2.1: The DISTSQL option generates a physical query plan for a query. Query plans provide information around SQL execution, which can be used to troubleshoot slow queries. For more information about distributed SQL queries, see the DistSQL section of our SQL Layer Architecture docs.

EXPLAIN (DISTSQL) generates a physical query plan that provides high level information about how a query will be executed:

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> EXPLAIN (DISTSQL) SELECT l_shipmode, AVG(l_extendedprice) FROM lineitem GROUP BY l_shipmode;
 automatic |                      url
-----------+----------------------------------------------
   true    | https://cockroachdb.github.io/distsqlplan...

To view the DistSQL Plan Viewer, point your browser to the URL provided:

EXPLAIN (DISTSQL)

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.

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

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

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> EXPLAIN SELECT * FROM kv WHERE v BETWEEN 4 AND 5;
  tree | field  |      description
+------+--------+-----------------------+
  scan |        |
       | table  | kv@primary
       | spans  | ALL
       | filter | (v >= 4) AND (v <= 5)
(4 rows)

If there were an index on v, CockroachDB would be able to avoid scanning the entire table:

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> CREATE INDEX v ON kv (v);
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> EXPLAIN SELECT * FROM kv WHERE v BETWEEN 4 AND 5;
 tree | field | description
------+-------+-------------
 scan |       |
      | table | kv@v
      | spans | /4-/6
(3 rows)

Now, only part of the index v is getting scanned, specifically the key range starting at (and including) 4 and stopping before 6.

See also



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