Investigate PG 12+ CTE materialization

[ukd1]

Re:bensheldon/good_job#65 (comment)

We need to investigate CTE materialization - we're expecting the default to be ok, but should test that vs explicitly on or off too and pick the fastest.

• test perf for default
• test perf for on
• test perf for off
• depending on results, decide if we push support min to PG 12, or introduce specific queries for > pg 12