pg_background contrib module proposal

Hi All,

I would like to take over pg_background patch and repost for
discussion and review.

Initially Robert Haas has share this for parallelism demonstration[1]
and abandoned later with
summary of open issue[2] with this pg_background patch need to be
fixed, most of them seems to be
addressed in core except handling of type exists without binary
send/recv functions and documentation.
I have added handling for types that don't have binary send/recv
functions in the attach patch and will
work on documentation at the end.

One concern with this patch is code duplication with
exec_simple_query(), we could
consider Jim Nasby’s patch[3] to overcome this, but certainly we
will end up by complicating
exec_simple_query() to make pg_background happy.

As discussed previously[1] pg_background is a contrib module that lets
you launch
arbitrary command in a background worker.

• VACUUM in background
• Autonomous transaction implementation better than dblink way (i.e.
no separate authentication required).
• Allows to perform task like CREATE INDEX CONCURRENTLY from a
procedural language.

This module comes with following SQL APIs:

• pg_background_launch : This API takes SQL command, which user wants
to execute, and size of queue buffer.
This function returns the process id of background worker.
• pg_background_result : This API takes the process id as input
parameter and returns the result of command
executed thought the background worker.
• pg_background_detach : This API takes the process id and detach the
background process which is waiting for
user to read its results.

Here's an example of running vacuum and then fetching the results.
Notice that the
notices from the original session are propagated to our session; if an
error had occurred,
it would be re-thrown locally when we try to read the results.

postgres=# create table foo (a int);
CREATE TABLE
postgres=# insert into foo values(generate_series(1,5));
INSERT 0 5

postgres=# select pg_background_launch('vacuum verbose foo');
pg_background_launch
----------------------
65427
(1 row)

postgres=# select * from pg_background_result(65427) as (x text);
INFO: vacuuming "public.foo"
INFO: "foo": found 0 removable, 5 nonremovable row versions in 1 out of 1 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
Skipped 0 pages due to buffer pins.
0 pages are entirely empty.
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.
x
--------
VACUUM
(1 row)

Thanks to Vibhor kumar, Rushabh Lathia and Robert Haas for feedback.

Please let me know your thoughts, and thanks for reading.

[1]. https://www.postgresql.org/message-id/CA%2BTgmoam66dTzCP8N2cRcS6S6dBMFX%2BJMba%2BmDf68H%3DKAkNjPQ%40mail.gmail.com
[2]. https://www.postgresql.org/message-id/CA%2BTgmobPiT_3Qgjeh3_v%2B8Cq2nMczkPyAYernF_7_W9a-6T1PA%40mail.gmail.com
[3]. https://www.postgresql.org/message-id/54541779.1010906%40BlueTreble.com

Regards,
Amul