On Fri, Mar 22, 2019 at 11:56 AM Tom Lane <t...@sss.pgh.pa.us> wrote: > In cases where, say, the first child requires no sort but also doesn't > emit very many rows, while the second child requires an expensive sort, > the planner will have a ridiculously optimistic opinion of the cost of > fetching slightly more rows than are available from the first child. > This might lead it to wrongly choose a merge join over a hash for example.
I think this is very much a valid point, especially in view of the fact that we already choose supposedly fast-start plans too often. I don't know whether it's a death sentence for this patch, but it should at least make us stop and think hard. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
