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We have long forbidden fetching backwards from a NO SCROLL cursor, but the prohibition didn't extend to cases in which we rewind the query altogether and then re-fetch forwards. I think the reason is that this logic was mainly meant to protect plan nodes that can't be run in the reverse direction. However, re-reading the query output is problematic if the query is volatile (which includes SELECT FOR UPDATE, not just queries with volatile functions): the re-read can produce different results, which confuses the cursor navigation logic completely. Another reason for disliking this approach is that some code paths will either fetch backwards or rewind-and-fetch-forwards depending on the distance to the target row; so that seemingly identical use-cases may or may not draw the "cursor can only scan forward" error. Hence, let's clean things up by disallowing rewind as well as fetch-backwards in a NO SCROLL cursor. Ordinarily we'd only make such a definitional change in HEAD, but there is a third reason to consider this change now. Commit |
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