I've got a customer that is running a pretty expensive function as part of a WHERE clause. With or without the function, the table the function references is the inner-most of a series of nested loops. Without the function things are very fast, but adding the function increases the cost of the index scan on that table by a factor of ~80x. It also falsely skews the row estimate further down, causing a bad shift to materialization in another part of the query, but that's a different problem. Wrapping the majority of the query in an OFFSET 0 with the function call on the outside makes things fast again.

It'd be nice if function execution could be delayed to a higher level of a query based on the cost.
Jim Nasby, Chief Data Architect, OpenSCG

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