Parallelism can block creation of columnar tables or views

[wuputah]

Example:

CREATE TABLE sampletable (x numeric);
INSERT INTO sampletable SELECT i FROM generate_series(1,1000000) i;

-- since there are some parallel workers reading from heap, and CommandCounterIncrement
-- is not expected to be called during parallel operations
CREATE TABLE sampletable_columnar USING columnar AS SELECT * FROM sampletable ORDER BY 1 ASC;
ERROR: cannot start commands during a parallel operation

In this case the workaround is to temporarily disable parallelism, e.g. set max_parallel_workers = 1.

[mausch]

This is preventing us from running https://github.com/moj-analytical-services/splink on Hydra.