PostgreSQL 9.0.23 Documentation
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# 42.6. Trigger Functions

When a function is used as a trigger, the dictionary `TD` contains trigger-related values:

`TD["event"]`

  contains the event as a string: `INSERT`, `UPDATE`, `DELETE`, `TRUNCATE`, or `UNKNOWN`.

`TD["when"]`

  contains one of `BEFORE`, `AFTER`, or `UNKNOWN`.

`TD["level"]`

  contains one of `ROW`, `STATEMENT`, or `UNKNOWN`.

`TD["new"]`
`TD["old"]`

  For a row-level trigger, one or both of these fields contain the respective trigger rows, depending on the trigger event.

`TD["name"]`

  contains the trigger name.

`TD["table_name"]`

  contains the name of the table on which the trigger occurred.

`TD["table_schema"]`

  contains the schema of the table on which the trigger occurred.

`TD["relid"]`

  contains the OID of the table on which the trigger occurred.

`TD["args"]`

  If the `CREATE TRIGGER` command included arguments, they are available in `TD["args"][0]` to `TD["args"][`n`-1]`.

If `TD["when"]` is `BEFORE` and `TD["level"]` is `ROW`, you can return `None` or `"OK"` from the Python function to indicate the row is unmodified, `"SKIP"` to abort the event, or `"MODIFY"` to indicate you've modified the row. Otherwise the return value is ignored.

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