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Message-ID: <7169c618-3975-4149-b3a5-00b26aef4268@aklaver.com>
Date: Tue, 11 Jun 2024 12:46:24 -0700
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Subject: Re: Does trigger only accept functions?
To: veem v <veema0000@gmail.com>, depesz@depesz.com
Cc: pgsql-general <pgsql-general@lists.postgresql.org>
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From: Adrian Klaver <adrian.klaver@aklaver.com>
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On 6/11/24 12:20, veem v wrote:
> 
> 
> On Wed, 12 Jun 2024 at 00:26, hubert depesz lubaczewski 
> <depesz@depesz.com <mailto:depesz@depesz.com>> wrote:
> 
> 
> 
>     No, I meant building dynamic queries and then EXECUTE-ing, like docs
>     show:
>     https://www.postgresql.org/docs/current/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN <https://www.postgresql.org/docs/current/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN>
> 
>     Best regards,
> 
>     depesz
> 
> 
> My apology, if interpreting it wrong way. It doesn't make much 
> difference though, but do you mean something like below?
> 
> CREATE OR REPLACE FUNCTION log_deletes()
> RETURNS TRIGGER AS $$
> DECLARE
>      audit_table_name TEXT;
>      audit_query TEXT;
> BEGIN
>      IF TG_TABLE_NAME = 'source_table1' THEN
>          audit_table_name := 'delete_audit1';
>          audit_query := 'INSERT INTO ' || audit_table_name || ' 
> (record_id, delete_timestamp, col1, col2, col3) VALUES ( $2, $3, $4)';
>          EXECUTE audit_query USING OLD.id, OLD.col1, OLD.col2, OLD.col3;
>      ELSIF TG_TABLE_NAME = 'source_table2' THEN
>          audit_table_name := 'delete_audit2';
>          audit_query := 'INSERT INTO ' || audit_table_name || ' ( col4, 
> col5, col6) VALUES ( $2, $3, $4)';
>          EXECUTE audit_query USING OLD.id, OLD.col4, OLD.col5, OLD.col6;
> 
>      ELSE
>          RAISE EXCEPTION 'Audit table not defined for %', TG_TABLE_NAME;
>      END IF;
> 
>      RETURN OLD;
> END;
> $$ LANGUAGE plpgsql;

I'm guessing depesz meant using TG_TABLE_NAME to pull column information 
from:

https://www.postgresql.org/docs/current/catalog-pg-attribute.html

and use that to build the INSERT query. The issue with dynamic or a 
fixed SQL is going to be with audit_query, in particular 
audit_table_name := 'delete_audit2. If your source tables change, add or 
delete columns or column types change, your audit table will need to 
change to match.

One possible solution is something I outlined here:

https://aklaver.org/wordpress/2021/12/07/postgres-and-json/

Other folks have done similar things, you can search on

postgresql audit tables using json

for alternatives.

-- 
Adrian Klaver
adrian.klaver@aklaver.com