In a bug report back in November [1] a subthread explored why parallel query is excluded any time we have "Plan nodes which reference a correlated SubPlan". Amit's understanding was that the reasoning had to do with inability to easily pass (potentially variable length) Param values between workers.
However a decent-sized subset of this kind of query doesn't actually require that we communicate between workers. If the Subplan executes per-tuple within the worker then there's no reason I can see why it needs to be marked parallel unsafe. Amit concurred but noted that identifying that subset of plans is the difficult part (as is usually the case!) At the time I'd started work on an approach to handle this case and hoped to "post about it in a new thread later this week." That didn't happen, but here we are now, and I finally have this patch cleaned up enough to share. The basic idea is that we need to track (both on nodes and relations) not only whether that node or rel is parallel safe but also whether it's parallel safe assuming params are rechecked in the using context. That allows us to delay making a final decision until we have sufficient context to conclude that a given usage of a Param is actually parallel safe or unsafe. The first patch in this series was previously posted in the thread "Consider parallel for lateral subqueries with limit" [2] and is required as a precursor for various test cases to work here. The second patch implements the core of the series. It results in parallel query being possible for subplans that execute entirely within the context of a parallel worker for cases where that subplan is in the target, a LATERAL JOIN, or the WHERE and ORDER BY clauses. The final patch notes several places where we set e.g. rel->consider_parallel but setting the corresponding new value rel->consider_parallel_recheckng_params wasn't yet necessary. It shows opportunity either for further improvement or concluding certain cases can't benefit and should be left unchanged. James 1: https://www.postgresql.org/message-id/CAAaqYe_vihKjc%2B8LuQa49EHW4%2BKfefb3wHqPYFnCuUqozo%2BLFg%40mail.gmail.com 2: https://www.postgresql.org/message-id/flat/CAAaqYe_HEkmLwf_1iEHxXwQOWiRyiFd%3DuOu6kwj3sWYdVd1-zA%40mail.gmail.com
v1-0003-Other-places-to-consider-for-completeness.patch
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v1-0002-Parallel-query-support-for-basic-correlated-subqu.patch
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v1-0001-Allow-parallel-LATERAL-subqueries-with-LIMIT-OFFS.patch
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