PostgreSQL 8.3.1 Documentation
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# 41.2. Trigger Functions

When a function is used as a trigger, the dictionary `TD` contains trigger-related values. The trigger rows are in `TD["new"]` and/or `TD["old"]` depending on the trigger event. `TD["event"]` contains the event as a string (`INSERT`, `UPDATE`, `DELETE`, or `UNKNOWN`). `TD["when"]` contains one of `BEFORE`, `AFTER`, and `UNKNOWN`. `TD["level"]` contains one of `ROW`, `STATEMENT`, and `UNKNOWN`. `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["name"]` contains the trigger name, and `TD["relid"]` contains the OID of the table on which the trigger occurred. If the `CREATE TRIGGER` command included arguments, they are available in `TD["args"][0]` to `TD["args"][(`n`-1)]`.

If `TD["when"]` is `BEFORE`, 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.

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