The EXPLAIN statement returns CockroachDB's query plan for an explainable statement. You can then use this information to optimize the query.
Explainable Statements
You can EXPLAIN on the following statements:
ALTER USER,ALTER TABLE,ALTER INDEX,ALTER VIEW,ALTER DATABASE,ALTER SEQUENCEBACKUPCANCEL JOB,CANCEL QUERYCREATE DATABASE,CREATE INDEX,CREATE TABLE,CREATE TABLE AS,CREATE USER,CREATE VIEW,CREATE SEQUENCEDELETEDROP DATABASE,DROP INDEX,DROP SEQUENCE,DROP TABLE,DROP USER,DROP VIEWEXECUTEEXPLAINIMPORTINSERTPAUSE JOBRESETRESTORERESUME JOBSELECTand any selection querySETSET CLUSTER SETTINGSHOW BACKUP,SHOW COLUMNS,SHOW CONSTRAINTS,SHOW CREATE TABLE,SHOW CREATE VIEW,SHOW CREATE SEQUENCE,SHOW CLUSTER SETTING,SHOW DATABASES,SHOW GRANTS,SHOW INDEX,SHOW JOBS,SHOW QUERIES,SHOW SESSIONS,SHOW TABLES,SHOW TRACE,SHOW USERS,SHOW HISTOGRAMUPDATEUPSERT
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
WHEREclause.
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
tableKey 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
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 statement you want details 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 |
|---|---|
| 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. This column displays only if the METADATA option is specified or implied. |
| 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. 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 index key ranges are used by a query:
> EXPLAIN SELECT * FROM kv WHERE v > 3 ORDER BY v;
+-----------+-------+-------------+
| Tree | Field | Description |
+-----------+-------+-------------+
| sort | | |
| │ | order | +v |
| └── scan | | |
| | table | kv@primary |
| | spans | ALL |
+-----------+-------+-------------+
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.
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;
+-----------+--------+-------------+
| Tree | Field | Description |
+-----------+--------+-------------+
| sort | | |
| │ | order | +v |
| └── scan | | |
| | table | kv@primary |
| | spans | ALL |
| | filter | v > 3 |
+-----------+--------+-------------+
METADATA Option
The METADATA option includes detail about which columns are being used by each
level, as well as properties of the result set on that level:
> EXPLAIN (METADATA) SELECT * FROM kv WHERE v > 3 ORDER BY v;
+-----------+-------+------+-------+-------------+---------+------------------------------+
| Tree | Level | Type | Field | Description | Columns | Ordering |
+-----------+-------+------+-------+-------------+---------+------------------------------+
| sort | 0 | sort | | | (k, v) | k!=NULL; v!=NULL; key(k); +v |
| │ | 0 | | order | +v | | |
| └── scan | 1 | scan | | | (k, v) | k!=NULL; v!=NULL; key(k) |
| | 1 | | table | kv@primary | | |
| | 1 | | spans | ALL | | |
+-----------+-------+------+-------+-------------+---------+------------------------------+
The Ordering column most importantly includes the ordering of the rows at
that level (+v in this case), but it also includes other information about the
result set at that level. In this case, CockroachDB was able to deduce that k
and v cannot be NULL, and k is a "key", meaning that you cannot have more
than one row with any given value of k.
Note that descending (DESC) orderings are indicated by the - sign:
> EXPLAIN (METADATA) SELECT * FROM kv WHERE v > 3 ORDER BY v DESC;
+-----------+-------+------+-------+-------------+---------+------------------------------+
| Tree | Level | Type | Field | Description | Columns | Ordering |
+-----------+-------+------+-------+-------------+---------+------------------------------+
| sort | 0 | sort | | | (k, v) | k!=NULL; v!=NULL; key(k); -v |
| │ | 0 | | order | -v | | |
| └── scan | 1 | scan | | | (k, v) | k!=NULL; v!=NULL; key(k) |
| | 1 | | table | kv@primary | | |
| | 1 | | spans | ALL | | |
+-----------+-------+------+-------+-------------+---------+------------------------------+
Another property that is reported in the Ordering column is information about columns that are known to be equal on any row, and "constant" columns that are known to have the same value on all rows. For example:
> EXPLAIN (METADATA) SELECT * FROM abcd JOIN efg ON a=e AND c=1;
+-----------+-------+------+----------------+--------------+-----------------------+-------------------------------+
| Tree | Level | Type | Field | Description | Columns | Ordering |
+-----------+-------+------+----------------+--------------+-----------------------+-------------------------------+
| join | 0 | join | | | (a, b, c, d, e, f, g) | a=e; c=CONST; a!=NULL; key(a) |
| │ | 0 | | type | inner | | |
| │ | 0 | | equality | (a) = (e) | | |
| │ | 0 | | mergeJoinOrder | +"(a=e)" | | |
| ├── scan | 1 | scan | | | (a, b, c, d) | c=CONST; a!=NULL; key(a); +a |
| │ | 1 | | table | abcd@primary | | |
| │ | 1 | | spans | ALL | | |
| └── scan | 1 | scan | | | (e, f, g) | e!=NULL; key(e); +e |
| | 1 | | table | efg@primary | | |
| | 1 | | spans | ALL | | |
+-----------+-------+------+----------------+--------------+-----------------------+-------------------------------+
This indicates that on any row, column a has the same value with column e,
and that all rows have the same value on column c.
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;
+----------------+----------+-------------+
| Tree | Field | Description |
+----------------+----------+-------------+
| render | | |
| │ | render 0 | a.v |
| │ | render 1 | b.v |
| └── join | | |
| │ | type | cross |
| ├── scan | | |
| │ | table | kv@primary |
| │ | spans | ALL |
| └── scan | | |
| | table | kv@primary |
| | spans | ALL |
+----------------+----------+-------------+
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 kv AS a, 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 | kv@primary |
| 2 | scan | | |
| 2 | | table | kv@primary |
+-------+--------+----------+-------------+
VERBOSE Option
The VERBOSE option is an alias for the combination of 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;
+---------------------+-------+--------+----------------+------------------+-----------------------+------------------------------+
| Tree | Level | Type | Field | Description | Columns | Ordering |
+---------------------+-------+--------+----------------+------------------+-----------------------+------------------------------+
| sort | 0 | sort | | | (k, v, v) | k!=NULL; key(k); -v |
| │ | 0 | | order | -v | | |
| └── render | 1 | render | | | (k, v, v) | k!=NULL; key(k) |
| │ | 1 | | render 0 | a.k | | |
| │ | 1 | | render 1 | a.v | | |
| │ | 1 | | render 2 | radu.public.kv.v | | |
| └── join | 2 | join | | | (k, v, k[omitted], v) | k=k; k!=NULL; key(k) |
| │ | 2 | | type | inner | | |
| │ | 2 | | equality | (k) = (k) | | |
| │ | 2 | | mergeJoinOrder | +"(k=k)" | | |
| ├── scan | 3 | scan | | | (k, v) | k!=NULL; v!=NULL; key(k); +k |
| │ | 3 | | table | kv@primary | | |
| │ | 3 | | spans | ALL | | |
| │ | 3 | | filter | v > 3 | | |
| └── scan | 3 | scan | | | (k, v) | k!=NULL; key(k); +k |
| | 3 | | table | kv@primary | | |
| | 3 | | spans | ALL | | |
+---------------------+-------+--------+----------------+------------------+-----------------------+------------------------------+
TYPES Option
The TYPES mode includes the types of the values used in the query plan, and implies the METADATA and EXPRS options as well:
> EXPLAIN (TYPES) SELECT * FROM kv WHERE v > 3 ORDER BY v;
+-----------+-------+------+--------+-----------------------------+----------------+------------------------------+
| Tree | Level | Type | Field | Description | Columns | Ordering |
+-----------+-------+------+--------+-----------------------------+----------------+------------------------------+
| sort | 0 | sort | | | (k int, v int) | k!=NULL; v!=NULL; key(k); +v |
| │ | 0 | | order | +v | | |
| └── scan | 1 | scan | | | (k int, v int) | k!=NULL; v!=NULL; key(k) |
| | 1 | | table | kv@primary | | |
| | 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 | kv@primary |
| 0 | | spans | ALL |
+-------+------+-------+-------------+
If there were an index on v, CockroachDB would be able to avoid scanning the
entire table:
> CREATE INDEX v ON kv (v);
> EXPLAIN SELECT * FROM kv WHERE v BETWEEN 4 AND 5;
+------+-------+-------------+
| Tree | Field | Description |
+------+-------+-------------+
| scan | | |
| | table | kv@v |
| | spans | /4-/6 |
+------+-------+-------------+
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
ALTER TABLEALTER SEQUENCEBACKUPCANCEL JOBCREATE DATABASEDROP DATABASEEXECUTEIMPORT- Indexes
INSERTPAUSE JOBRESETRESTORERESUME JOBSELECT- Selection Queries
SETSET CLUSTER SETTINGSHOW COLUMNSUPDATEUPSERT
Was this helpful?
