Subqueries

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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

New in v19.1: 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 following exceptions:

• Correlated subqueries that generate side effects inside a CASE statement.

• Correlated subqueries that result in implicit LATERAL joins. Given a cross-join expression a,b, if b is an application of a set-returning function that references a variable defined in the surrounding query, the LATERAL keyword is assumed as shown below.

  > SELECT
        e.last_name, s.salary, noise
    FROM
        employees AS e,
        salaries AS s,
        -- Join with a set-returning function implies LATERAL below
        generate_series(0, s.salary, 10000) AS noise
    WHERE
        e.emp_no = s.emp_no
    ORDER BY
        s.salary DESC
    LIMIT
        10;
  

  ERROR:  no data source matches prefix: s
  

  For more information, see the Github issue tracking LATERAL join implementation.

  Note that the example above uses the employees data set that is also used in our Migrate from MySQL instructions (and the MySQL docs).

Performance best practices

• 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

• Selection Queries
• Scalar Expressions
• Table Expressions
• Performance Best Practices - Overview

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