Race condition in partitioning with dynamic table creation - postgresql

Question

I'm trying to implement table partitioning with dynamic table creation using BEFORE INSERT trigger to create new tables and indexes when necesarry using following solution:
create table mylog (
mylog_id serial not null primary key,
ts timestamp(0) not null default now(),
data text not null
);
CREATE OR REPLACE FUNCTION mylog_insert() RETURNS trigger AS
$BODY$
DECLARE
_name text;
_from timestamp(0);
_to timestamp(0);
BEGIN
SELECT into _name 'mylog_'||replace(substring(date_trunc('day', new.ts)::text from 0 for 11), '-', '');
IF NOT EXISTS (SELECT 1 FROM information_schema.tables WHERE table_name=_name) then
SELECT into _from date_trunc('day', new.ts)::timestamp(0);
SELECT into _to _from + INTERVAL '1 day';
EXECUTE 'CREATE TABLE '||_name||' () INHERITS (mylog)';
EXECUTE 'ALTER TABLE '||_name||' ADD CONSTRAINT ts_check CHECK (ts >= '||quote_literal(_from)||' AND ts < '||quote_literal(_to)||')';
EXECUTE 'CREATE INDEX '||_name||'_ts_idx on '||_name||'(ts)';
END IF;
EXECUTE 'INSERT INTO '||_name||' (ts, data) VALUES ($1, $2)' USING
new.ts, new.data;
RETURN null;
END;
$BODY$
LANGUAGE plpgsql;
CREATE TRIGGER mylog_insert
BEFORE INSERT
ON mylog
FOR EACH ROW
EXECUTE PROCEDURE mylog_insert();
Everything works as expected but each day when concurrent INSERT statements are being fired for the first time that day, one of them fails trying to "create table that already exists". I suspect that this is caused by the triggers being fired concurrently and both trying to create new table and only one can succeed.
I could be using CREATE TABLE IF NOT EXIST but I cannot detect the outcome so I cannot reliably create constraints and indexes.
What can I do to avoid such problem? Is there any way to signal the fact that the table has been already created to other concurrent triggers? Or maybe there is a way of knowing if CREATE TABLE IF NOT EXISTS created new table or not?

Answer 1

What I do is create a pgAgent job to run every day and create 3 months of tables ahead of time.
CREATE OR REPLACE FUNCTION avl_db.create_alltables()
RETURNS numeric AS
$BODY$
DECLARE
rec record;
BEGIN
FOR rec IN
SELECT date_trunc('day', i::timestamp without time zone) as table_day
FROM generate_series(now()::date,
now()::date + '3 MONTH'::interval,
'1 DAY'::interval) as i
LOOP
PERFORM avl_db.create_table (rec.table_day);
END LOOP;
PERFORM avl_db.avl_partition(now()::date,
now()::date + '3 MONTH'::interval);
RETURN 0;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION avl_db.create_alltables()
OWNER TO postgres;
create_table is very similar to your CREATE TABLE code
avl_partition update the BEFORE INSERT TRIGGER but I saw you do that part with dynamic query. Will have to check again that.
Also I see you are doing inherit, but you are missing a very important CONSTRAINT
CONSTRAINT route_sources_20170601_event_time_check CHECK (
event_time >= '2017-06-01 00:00:00'::timestamp without time zone
AND event_time < '2017-06-02 00:00:00'::timestamp without time zone
)
This improve the query a lot when doing a search for event_time because doesn't have to check every table.
See how doesn't check all tables for the month:

Answer 2

Eventually I wrapped CREATE TABLE in BEGIN...EXCEPTION block that catches duplicate_table exception - this did the trick, but creating the tables upfront in a cronjob is much better approach performance-wise.
CREATE OR REPLACE FUNCTION mylog_insert() RETURNS trigger AS
$BODY$
DECLARE
_name text;
_from timestamp(0);
_to timestamp(0);
BEGIN
SELECT into _name 'mylog_'||replace(substring(date_trunc('day', new.ts)::text from 0 for 11), '-', '');
IF NOT EXISTS (SELECT 1 FROM information_schema.tables WHERE table_name=_name) then
SELECT into _from date_trunc('day', new.ts)::timestamp(0);
SELECT into _to _from + INTERVAL '1 day';
BEGIN
EXECUTE 'CREATE TABLE '||_name||' () INHERITS (mylog)';
EXECUTE 'ALTER TABLE '||_name||' ADD CONSTRAINT ts_check CHECK (ts >= '||quote_literal(_from)||' AND ts < '||quote_literal(_to)||')';
EXECUTE 'CREATE INDEX '||_name||'_ts_idx on '||_name||'(ts)';
EXCEPTION WHEN duplicate_table THEN
RAISE NOTICE 'table exists -- ignoring';
END;
END IF;
EXECUTE 'INSERT INTO '||_name||' (ts, data) VALUES ($1, $2)' USING
new.ts, new.data;
RETURN null;
END;
$BODY$
LANGUAGE plpgsql;