Fix #328 - Dont do limit pushdown during parallel execution

[noahisaksen]

#328

LIMIT pushdown was added to this extension. #313

It pushes LIMIT down to Postgres and removes it from the DuckDB query plan.
Problem is that parallelism can occur in the extension, and one Query gets turned into n postgres queries. Pushing LIMIT down then falsifies the result.

A test is added, using set pg_pages_per_task=1, that reproduces this by forcing parallelism.

I added a check on max_threads, which makes it so that LIMIT is NOT pushed down, if it is anything other than 1.
This should prevent LIMIT from being pushed down in any case where we would have parallel queries from the best of my knowledge. (There could be a better way to check for parallelism that I am not aware of)

Previously

set pg_pages_per_task=1;
FROM s.large_tbl LIMIT 1;

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(0,0)'::tid AND '(1,0)'::tid LIMIT 1) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(1,0)'::tid AND '(2,0)'::tid LIMIT 1) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(2,0)'::tid AND '(3,0)'::tid LIMIT 1) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(3,0)'::tid AND '(4,0)'::tid LIMIT 1) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(4,0)'::tid AND '(5,0)'::tid LIMIT 1) TO STDOUT (FORMAT "binary");

┌────────────┐
│     i      │
│   int64    │
├────────────┤
│      70286 │
│      72772 │
│      74128 │
│      75032 │
│      76388 │
│      79100 │

AFTER (NO LIMIT DURING PARALLEL)

set pg_pages_per_task=1;
FROM s.large_tbl LIMIT 1;

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(0,0)'::tid AND '(1,0)'::tid) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(1,0)'::tid AND '(2,0)'::tid) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(2,0)'::tid AND '(3,0)'::tid) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(3,0)'::tid AND '(4,0)'::tid) TO STDOUT (FORMAT "binary");

COPY (SELECT "i" FROM "public"."large_tbl" WHERE ctid BETWEEN '(4,0)'::tid AND '(5,0)'::tid) TO STDOUT (FORMAT "binary");

┌───────┐
│   i   │
│ int64 │
├───────┤
│   0   │
└───────┘