Getting the exact SQL from inside an event trigger

From: "Joe Wildish" <joe(at)lateraljoin(dot)com>
To: pgsql-general(at)lists(dot)postgresql(dot)org
Subject: Getting the exact SQL from inside an event trigger
Date: 2023-03-02 11:12:37
Message-ID: 10164c95-ef23-4624-9251-ea6ff594cd1c@app.fastmail.com
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Hello all,

We are using event triggers to capture DDL for subsequent replay on a logical replica.

The intention is to write the DDL statement to a table, inside the same transaction that executes the DDL, and have a separate process on the replica notice changes in this table and execute whatever it finds.

We have declared a trigger on the ddl_command_end event for this purpose. We can get the SQL from running current_query() inside the trigger; oddly, the pg_event_trigger_ddl_commands() function does have an attribute called "command", typed as "pg_ddl_command", but there are no SQL functions that can operate on this type, including turning it into a string.

This process works for a simple case of e.g. "CREATE TABLE t()".

However, in other cases --- e.g. "DO $$ BEGIN CREATE TABLE t(); CREATE TABLE s(); END; $$;" --- the trigger will fire for each CREATE TABLE but the current_query() will evaluate to the entire DO block.

This makes it difficult to capture just the actual statement that is being executed. I am looking for a way to get the precise statement that is being executed from within the ddl_command_event trigger function. Does anyone know if this is possible?

Thanks,
-Joe

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