SQL subqueries enable reuse of the results from a selection query within another query.
Overview
CockroachDB supports two kinds of subqueries:
- Relational subqueries which appear as operand in selection queries or table expressions.
- Scalar subqueries which appear as operand in a scalar expression.
Data writes in subqueries
When a subquery contains a data-modifying statement (INSERT,
DELETE, etc.), the data modification is always executed to
completion even if the surrounding query only uses a subset of the
result rows.
This is true both for subqueries defined using the (...) or [...]
notations, and those defined using
WITH.
For example:
> SELECT *
FROM [INSERT INTO t(x) VALUES (1), (2), (3) RETURNING x]
LIMIT 1;
This query always inserts 3 rows into t, even though the surrounding
query only observes 1 row using LIMIT.
Correlated subqueries
CockroachDB's cost-based optimizer supports most correlated subqueries.
A subquery is said to be "correlated" when it uses table or column names defined in the surrounding query.
For example, to find every customer with at least one order, run:
> SELECT
c.name
FROM
customers AS c
WHERE
EXISTS(
SELECT * FROM orders AS o WHERE o.customer_id = c.id
);
The subquery is correlated because it uses c defined in the surrounding query.
Limitations
The cost-based optimizer supports most correlated subqueries, with the exception of correlated subqueries that generate side effects inside a CASE statement.
Performance best practices
CockroachDB is currently undergoing major changes to evolve and improve the performance of subqueries. The restrictions and workarounds listed in this section will be lifted or made unnecessary over time.
Scalar subqueries currently disable the distribution of the execution of a query. To ensure maximum performance on queries that process a large number of rows, make the client application compute the subquery results ahead of time and pass these results directly in the surrounding query.
The results of scalar subqueries are currently loaded entirely into memory when the execution of the surrounding query starts. To prevent execution errors due to memory exhaustion, ensure that subqueries return as few results as possible.
See also
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