Performance of tqueue.c's tuple remapping logic

$SUBJECT sucks.

Create a table containing lots of composite arrays:

regression=# create table foo (f1 int8_tbl[]);
CREATE TABLE
regression=# insert into foo select array[row(1,2),row(3,4)]::int8_tbl[] from generate_series (1,10000000);
INSERT 0 10000000
regression=# vacuum analyze foo;
VACUUM

Establish a baseline for how long it takes to scan this table:

regression=# explain analyze select f1 from foo;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
Seq Scan on foo (cost=0.00..263935.06 rows=10000006 width=101) (actual time=0.027..1461.236 rows=10000000 loops=1)
Planning time: 0.149 ms
Execution time: 1996.995 ms
(3 rows)

... or select a non-composite value out of it:

regression=# explain analyze select f1[1].q1 from foo;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
Seq Scan on foo (cost=0.00..263935.06 rows=10000006 width=8) (actual time=1.122..3736.121 rows=10000000 loops=1)
Planning time: 0.077 ms
Execution time: 4285.872 ms
(3 rows)

Now let's try those same queries in parallel mode:

regression=# set force_parallel_mode to 1;
SET
regression=# explain analyze select f1[1].q1 from foo;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Gather (cost=1000.00..1264935.66 rows=10000006 width=8) (actual time=11.402..12753.782 rows=10000000 loops=1)
Workers Planned: 1
Workers Launched: 1
Single Copy: true
-> Seq Scan on foo (cost=0.00..263935.06 rows=10000006 width=8) (actual time=0.182..4523.724 rows=10000000 loops=1)
Planning time: 0.081 ms
Execution time: 13736.321 ms
(7 rows)

regression=# explain analyze select f1 from foo;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
Gather (cost=1000.00..1264935.66 rows=10000006 width=101) (actual time=6.659..22693.798 rows=10000000 loops=1)
Workers Planned: 1
Workers Launched: 1
Single Copy: true
-> Seq Scan on foo (cost=0.00..263935.06 rows=10000006 width=101) (actual time=0.780..2579.707 rows=10000000 loops=1)
Planning time: 0.073 ms
Execution time: 25925.709 ms
(7 rows)

So, having to do record-type remapping in the fairly trivial case of
two-element composite arrays nearly doubles the already rather excessive
runtime for a parallel query returning lots of data.

Just to add insult to injury, the backend's memory consumption bloats
to something over 5.5G during that last query. Which is not terribly
surprising given all the cavalier use of TopMemoryContext in tqueue.c.

Since the entire point of parallel query is to deal with large data
volumes, I think this means that tqueue.c is simply unfit for production
use as it stands.

I propose to undertake a thorough code review, and possibly significant
rewrite, of tqueue.c over the next few days.

regards, tom lane