As proposed the complete changes to pg_trigger and pg_rewrite

For discussion:

Attached is the completed patch that changes pg_trigger and extends
pg_rewrite in order to allow triggers and rules to be defined with
different, per session controllable, behaviors for replication purposes.

This will allow replication systems like Slony-I and, as has been stated
on pgsql-hackers, other products to control the firing mechanism of
triggers and rewrite rules without modifying the system catalog directly.

The firing mechanisms are controlled by a new superuser-only GUC
variable, session_replication_role, together with a change to
pg_trigger.tgenabled and a new column pg_rewrite.ev_enabled. Both
columns are a single char data type now (tgenabled was a bool before).
The possible values in these attributes are:

'O' - Trigger/Rule fires when session_replication_role is "origin"
(default) or "local". This is the default behavior.

'D' - Trigger/Rule is disabled and fires never

'A' - Trigger/Rule fires always regardless of the setting of
session_replication_role

'R' - Trigger/Rule fires when session_replication_role is "replica"

The GUC variable can only be changed as long as the system does not have
any saved SPI plans. This will prevent changing the session role and
accidentally executing stored procedures or functions that have plans
cached that expand to the wrong query set due to differences in the rule
firing semantics.

The SQL syntax for changing a triggers/rules firing semantics is

ALTER TABLE <tabname> <when> TRIGGER|RULE <name>;

<when> ::= ENABLE | ENABLE ALWAYS | ENABLE REPLICA | DISABLE

psql's \d command as well as pg_dump are extended in a backward
compatible fashion.

Any volunteers to do the corresponding documentation changes should this
patch be accepted?

Jan

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