> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/reference/sql-commands/sql-listen.md). # LISTEN ### LISTEN LISTEN — listen for a notification ### Synopsis ``` LISTEN channel ``` ### Description `LISTEN` registers the current session as a listener on the notification channel named *`channel`*. If the current session is already registered as a listener for this notification channel, nothing is done. Whenever the command `NOTIFY channel` is invoked, either by this session or another one connected to the same database, all the sessions currently listening on that notification channel are notified, and each will in turn notify its connected client application. A session can be unregistered for a given notification channel with the `UNLISTEN` command. A session's listen registrations are automatically cleared when the session ends. The method a client application must use to detect notification events depends on which PostgreSQL application programming interface it uses. With the libpq library, the application issues `LISTEN` as an ordinary SQL command, and then must periodically call the function `PQnotifies` to find out whether any notification events have been received. Other interfaces such as libpgtcl provide higher-level methods for handling notify events; indeed, with libpgtcl the application programmer should not even issue `LISTEN` or `UNLISTEN` directly. See the documentation for the interface you are using for more details. ### Parameters *`channel`* : Name of a notification channel (any identifier). ### Notes `LISTEN` takes effect at transaction commit. If `LISTEN` or `UNLISTEN` is executed within a transaction that later rolls back, the set of notification channels being listened to is unchanged. A transaction that has executed `LISTEN` cannot be prepared for two-phase commit. There is a race condition when first setting up a listening session: if concurrently-committing transactions are sending notify events, exactly which of those will the newly listening session receive? The answer is that the session will receive all events committed after an instant during the transaction's commit step. But that is slightly later than any database state that the transaction could have observed in queries. This leads to the following rule for using `LISTEN`: first execute (and commit!) that command, then in a new transaction inspect the database state as needed by the application logic, then rely on notifications to find out about subsequent changes to the database state. The first few received notifications might refer to updates already observed in the initial database inspection, but this is usually harmless. [NOTIFY](/reference/sql-commands/sql-notify.md) contains a more extensive discussion of the use of `LISTEN` and `NOTIFY`. ### Examples Configure and execute a listen/notify sequence from psql: ``` LISTEN virtual; NOTIFY virtual; Asynchronous notification "virtual" received from server process with PID 8448. ``` ### Compatibility There is no `LISTEN` statement in the SQL standard. ### See Also [NOTIFY](/reference/sql-commands/sql-notify.md), [UNLISTEN](/reference/sql-commands/sql-unlisten.md), [max\_notify\_queue\_pages](/server-administration/runtime-config/runtime-config-resource.md#GUC-MAX-NOTIFY-QUEUE-PAGES) *** 原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/sql-listen.html)（英文原文，待翻譯） --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.postgresql.tw/reference/sql-commands/sql-listen.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.