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select-for-update.md

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SELECT FOR UPDATE
The SELECT FOR UPDATE statement is used to order transactions under contention.
concurrency control, locking, transactions, update locking, update, contention
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{% include {{page.version.version}}/sql/select-for-update-overview.md %}

Synopsis

{% include {{ page.version.version }}/sql/diagrams/simple_select_clause.html %}

Required privileges

The user must have the SELECT and UPDATE privileges on the tables used as operands.

Parameters

The same as for other selection queries.

Examples

Enforce transaction order when updating the same rows

In this example, we'll use SELECT ... FOR UPDATE to lock a row inside a transaction, forcing other transactions that want to update the same row to wait for the first transaction to complete. The other transactions that want to update the same row are effectively put into a queue based on when they first try to read the value of the row.

This example assumes you are running a local unsecured cluster.

First, let's connect to the running cluster (we'll call this Terminal 1):

{% include copy-clipboard.html %}

cockroach sql --insecure

Next, let's create a table and insert some rows:

{% include copy-clipboard.html %}

CREATE TABLE kv (k INT PRIMARY KEY, v INT);
INSERT INTO kv (k, v) VALUES (1, 5), (2, 10), (3, 15);

Next, we'll start a transaction and and lock the row we want to operate on:

{% include copy-clipboard.html %}

BEGIN;
SELECT * FROM kv WHERE k = 1 FOR UPDATE;

Hit enter twice in the SQL client to send the input so far to be evaluated. This will result in the following output:

  k | v
+---+----+
  1 | 5
(1 row)

Now let's open another terminal and connect to the database from a second client (we'll call this Terminal 2):

{% include copy-clipboard.html %}

cockroach sql --insecure

From Terminal 2, start a transaction and try to lock the same row for updates that is already being accessed by the transaction we opened in Terminal 1:

{% include copy-clipboard.html %}

BEGIN;
SELECT * FROM kv WHERE k = 1 FOR UPDATE;

Hit enter twice to send the input so far to be evaluated. Because Terminal 1 has already locked this row, the SELECT ... FOR UPDATE statement from Terminal 2 will appear to "wait".

Back in Terminal 1, let's update the row and commit the transaction:

{% include copy-clipboard.html %}

UPDATE kv SET v = v + 5 WHERE k = 1;
COMMIT;
COMMIT

Now that the transaction in Terminal 1 has committed, the transaction in Terminal 2 will be "unblocked", generating the following output, which shows the value left by the transaction in Terminal 1:

  k | v
+---+----+
  1 | 10
(1 row)

The transaction in Terminal 2 can now receive input, so let's update the row in question again:

{% include copy-clipboard.html %}

UPDATE kv SET v = v + 5 WHERE k = 1;
COMMIT;
UPDATE 1

Finally, we commit the transaction in Terminal 2:

{% include copy-clipboard.html %}

COMMIT;
COMMIT

See also