Re: COMMIT TRIGGERs, take n, implemented with CONSTRAINT TRIGGERS

From: Nico Williams <nico(at)cryptonector(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: COMMIT TRIGGERs, take n, implemented with CONSTRAINT TRIGGERS
Date: 2017-09-15 20:00:35
Message-ID: 20170915200033.GC26093@localhost
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On Fri, Sep 15, 2017 at 12:25:08PM -0700, Andres Freund wrote:
> On 2017-09-15 14:19:29 -0500, Nico Williams wrote:
> > Please see my post and the linked file to see why.
>
> The discussions here are often going to be referred back to in years, so
> external links where we aren't sure about the longevity (like e.g. links
> to the mailing list archive, where we're fairly sure), aren't liked
> much. If you want to argue for a change, it should happen on-list.

Fair enough. I thought I had given enough detail, but here is the code.
It's just an event trigger that ensures every table has a DEFERRED
CONSTRAINT TRIGGER that runs a function that debounces invocations so
that the "commit trigger" function runs just once:

/*
* Copyright (c) 2017 Two Sigma Open Source, LLC.
* All Rights Reserved
*
* Permission to use, copy, modify, and distribute this software and its
* documentation for any purpose, without fee, and without a written agreement
* is hereby granted, provided that the above copyright notice and this
* paragraph and the following two paragraphs appear in all copies.
*
* IN NO EVENT SHALL TWO SIGMA OPEN SOURCE, LLC BE LIABLE TO ANY PARTY FOR
* DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
* LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION,
* EVEN IF TWO SIGMA OPEN SOURCE, LLC HAS BEEN ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* TWO SIGMA OPEN SOURCE, LLC SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
* BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS"
* BASIS, AND TWO SIGMA OPEN SOURCE, LLC HAS NO OBLIGATIONS TO PROVIDE
* MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
*/

/*
* This file demonstrates how to create a "COMMIT TRIGGER" for
* PostgreSQL using CONSTRAINT TRIGGERs.
*
* There have been many threads on the PG mailing lists about commit
* triggers, with much skepticism shown about the possible semantics of
* such a thing.
*
* Below we demonstrate reasonable, useful, and desirable semantics, how
* to obtain them with PG today.
*
* There are three shortcomings of this implementation:
*
* a) It is possible defeat this implementation by using
*
* SET CONSTRAINTS ... IMMEDIATE;
*
* or otherwise disabling the triggers created under the hood herein.
*
* The ability to make these triggers run early can be *dangerous*,
* depending on the application. It is especially dangerous given
* that no privilege is needed in order to do this, and there's no
* way for a CONSTRAINT TRIGGER to detect when it is called _last_,
* only when it is called _first_, in any transaction.
*
* b) This implementation serializes write transactions implicitly by
* having a single row encode commit trigger state.
*
* (This is easily fixed though.)
*
* c) This implementation is inefficient because CONSTRAINT TRIGGERs
* have to be FOR EACH ROW triggers. Thus a transaction that does
* 1,000 inserts will cause 999 unnecessary trigger procedure calls
* under the hood. Also, because CONSTRAINT TRIGGERs have to be FOR
* EACH ROW triggers, PG has to track OLD/NEW row values for all
* affected rows, even though commit triggers obviously don't need
* this.
*
* (Also, for simplicity we use SECURITY DEFINER functions here,
* otherwise we'd have to have additional code to grant to
* public the ability to call our functions. We would need additional
* code by which to ensure that users do not toggle internal state to
* prevent commit trigger execution.)
*
* For example, to create a commit trigger that invokes
* commit_trigger.example_proc() at the end of any _write_ transaction,
* run the following in psql:
*
* -- Load commit trigger functionality:
* \i commit_trigger.sql
*
* -- CREATE COMMIT TRIGGER egt
* -- EXECUTE PROCEDURE commit_trigger.example_proc();
* INSERT INTO commit_trigger.triggers
* (trig_name, proc_schema, proc_name)
* SELECT 'egt', 'commit_trigger', 'example_proc';
*
* Demo:
*
* db=# \i commit_trigger.sql
* <noise>
* db=# INSERT INTO commit_trigger.triggers
* db-# (trig_name, proc_schema, proc_name)
* db-# SELECT 'egt', 'commit_trigger', 'example_proc';
* db=#
* db=# CREATE SCHEMA eg;
* CREATE SCHEMA
* db=# CREATE TABLE eg.x(a text primary key);
* CREATE TABLE
* db=# BEGIN;
* BEGIN
* db=# INSERT INTO eg.x (a) VALUES('foo');
* INSERT 0 1
* db=# INSERT INTO eg.x (a) VALUES('bar');
* INSERT 0 1
* db=# COMMIT;
* NOTICE: example_proc() here! Should be just one for this TX (txid 208036)
* CONTEXT: PL/pgSQL function example_proc() line 3 at
* RAISE
* COMMIT
* db=# INSERT INTO eg.x (a) VALUES('foobar');
* NOTICE: example_proc() here! Should be just one for this TX (txid 208037)
* CONTEXT: PL/pgSQL function example_proc() line 3 at
* db=# INSERT INTO eg.x (a) VALUES('baz');
* NOTICE: example_proc() here! Should be just one for this TX (txid 208038)
* CONTEXT: PL/pgSQL function example_proc() line 3 at
* db=#
*
* Semantics:
*
* - commit trigger procedures called exactly once per-transaction that
* had any writes (even if they changed nothing in the end)
*
* (*Unless* someone first runs SET CONSTRAINTS ALL IMMEDIATE!)
*
* - commit trigger procedures called in order of commit trigger name
* (ascending)
*
* - commit trigger procedures may perform additional write operations,
* and if so that will NOT cause additional invocations of commit
* trigger procedures
*
* - commit trigger procedures may RAISE EXCEPTION, triggering a
* rollback of the transaction
*
* The above semantics are exactly what would be desired of a properly-
* integrated COMMIT TRIGGER feature, except that it SHOULD NEVER be
* possible to cause commit triggers to fire early by executing
* SET CONSTRAINTS ALL IMMEDIATE.
*/

\set ON_ERROR_STOP on

CREATE SCHEMA IF NOT EXISTS commit_trigger;

CREATE TABLE IF NOT EXISTS commit_trigger.triggers (
trig_name TEXT PRIMARY KEY,
proc_schema TEXT NOT NULL,
proc_name TEXT NOT NULL
);

/* State needed to prevent more than one commit trigger call per-commit */
CREATE TABLE IF NOT EXISTS commit_trigger.commit_trigger_called (
_id BIGINT PRIMARY KEY CHECK(_id = 0) DEFAULT(0),
_txid BIGINT CHECK(_txid = txid_current()) DEFAULT(txid_current()))
;

INSERT INTO commit_trigger.commit_trigger_called
SELECT
ON CONFLICT DO NOTHING;

/* Example commit trigger procesdure */
CREATE OR REPLACE FUNCTION commit_trigger.example_proc()
RETURNS VOID AS $$
BEGIN
RAISE NOTICE
'example_proc() here! Should be just one for this TX (txid %)',
txid_current();
END $$ LANGUAGE PLPGSQL SECURITY INVOKER SET search_path = commit_trigger;

CREATE OR REPLACE VIEW commit_trigger.synthetic_triggers AS
SELECT rn.nspname AS tbl_schema,
r.relname AS tbl_name,
coalesce(t.tgname,
'zzz_commit_trigger_' || rn.nspname || '_' || r.relname) AS tg_name,
t.tgenabled AS tg_enabled
FROM pg_catalog.pg_class r
JOIN pg_catalog.pg_namespace rn ON rn.oid = r.relnamespace
LEFT JOIN pg_trigger t ON t.tgrelid = r.oid
WHERE r.relkind = 'r' AND
(t.tgname IS NULL OR t.tgname LIKE 'zzz\_commit\_trigger\_%') AND
rn.nspname NOT IN ('commit_trigger', 'pg_catalog');

CREATE OR REPLACE FUNCTION commit_trigger.invoke_commit_triggers()
RETURNS VOID AS $$
DECLARE
t record;
BEGIN
FOR t IN (
SELECT ct.proc_schema AS proc_schema, proc_name AS proc_name
FROM commit_trigger.triggers ct
JOIN pg_catalog.pg_proc p ON ct.proc_name = p.proname
JOIN pg_catalog.pg_namespace pn ON p.pronamespace = pn.oid AND
pn.nspname = ct.proc_schema
ORDER BY trig_name ASC)
LOOP
EXECUTE format($q$
SELECT %1$I.%2$I();
$q$, t.proc_schema, t.proc_name);
END LOOP;
END $$ LANGUAGE PLPGSQL SECURITY INVOKER SET search_path = commit_trigger;

CREATE OR REPLACE FUNCTION commit_trigger.trig_proc()
RETURNS TRIGGER AS $$
BEGIN
IF NOT EXISTS(
SELECT *
FROM commit_trigger.commit_trigger_called
WHERE _txid = txid_current()) THEN
/*RAISE NOTICE 'Calling commit triggers (txid = %)', txid_current();*/
PERFORM commit_trigger.invoke_commit_triggers();
UPDATE commit_trigger.commit_trigger_called
SET _txid = txid_current();
END IF;
RETURN CASE TG_OP
WHEN 'INSERT' THEN NEW
WHEN 'UPDATE' THEN NEW
ELSE OLD
END;
END $$ LANGUAGE PLPGSQL SECURITY INVOKER SET search_path = commit_trigger;

CREATE OR REPLACE FUNCTION commit_trigger.make_triggers()
RETURNS void AS $$
DECLARE
t record;
BEGIN
FOR t IN (
SELECT st.tg_name AS tg_name,
st.tbl_schema AS tbl_schema,
st.tbl_name AS tbl_name
FROM commit_trigger.synthetic_triggers st
WHERE st.tg_enabled IS NULL)
LOOP
EXECUTE format($q$
CREATE CONSTRAINT TRIGGER %1$I
AFTER INSERT OR UPDATE OR DELETE
ON %2$I.%3$I
INITIALLY DEFERRED FOR EACH ROW
EXECUTE PROCEDURE commit_trigger.trig_proc();
$q$, t.tg_name, t.tbl_schema, t.tbl_name);
END LOOP;
DELETE FROM commit_trigger.triggers ct
WHERE NOT EXISTS (
SELECT p.*
FROM pg_catalog.pg_proc p
JOIN pg_catalog.pg_namespace pn ON p.pronamespace = pn.oid
WHERE pn.nspname = ct.proc_schema AND p.proname = ct.proc_name
);
END $$ LANGUAGE PLPGSQL SECURITY DEFINER SET search_path = commit_trigger;

CREATE OR REPLACE FUNCTION commit_trigger.event_make_triggers()
RETURNS event_trigger AS $$
BEGIN
PERFORM commit_trigger.make_triggers();
END $$ LANGUAGE PLPGSQL SECURITY DEFINER SET search_path = commit_trigger;

/*
* Make sure we define our internal triggers for all future new TABLEs,
* and that we cleanup when commit trigger procedures are DROPped.
*/
DROP EVENT TRIGGER IF EXISTS commit_trigger_make_triggers;
CREATE EVENT TRIGGER commit_trigger_make_triggers ON ddl_command_end
WHEN tag IN ('CREATE TABLE', 'DROP FUNCTION')
EXECUTE PROCEDURE commit_trigger.event_make_triggers();

/* Create our internal triggers for all existing tables now */
SELECT commit_trigger.make_triggers();

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