> 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/server-programming/spi/spi-visibility.md).

# 45.5. Visibility of Data Changes \#

The following rules govern the visibility of data changes in functions that use SPI (or any other C function):

* During the execution of an SQL command, any data changes made by the command are invisible to the command itself. For example, in:

  ```

  INSERT INTO a SELECT * FROM a;
  ```

  the inserted rows are invisible to the `SELECT` part.
* Changes made by a command C are visible to all commands that are started after C, no matter whether they are started inside C (during the execution of C) or after C is done.
* Commands executed via SPI inside a function called by an SQL command (either an ordinary function or a trigger) follow one or the other of the above rules depending on the read/write flag passed to SPI. Commands executed in read-only mode follow the first rule: they cannot see changes of the calling command. Commands executed in read-write mode follow the second rule: they can see all changes made so far.
* All standard procedural languages set the SPI read-write mode depending on the volatility attribute of the function. Commands of `STABLE` and `IMMUTABLE` functions are done in read-only mode, while commands of `VOLATILE` functions are done in read-write mode. While authors of C functions are able to violate this convention, it's unlikely to be a good idea to do so.

The next section contains an example that illustrates the application of these rules.

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/spi-visibility.html)（英文原文，待翻譯）


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