5ac9c43073
There was some code in gen_prune_steps_from_opexps that needlessly checked a list was not empty when it clearly had to contain at least one item. This prompted a further cleanup operation in partprune.c. Additionally, the previous code could end up adding additional needless INTERSECT steps. However, those do not appear to be able to cause any misbehavior. gen_prune_steps_from_opexps is now no longer in charge of generating combine pruning steps. Instead, gen_partprune_steps_internal, which already does some combine step creation has been given the sole responsibility of generating all combine steps. This means that when we recursively call gen_partprune_steps_internal, since it always now adds a combine step when it produces multiple steps, we can just pay attention to the final step returned. In passing, do quite a bit of work on the comments to try to more clearly explain the role of both gen_partprune_steps_internal and gen_prune_steps_from_opexps. This is fairly complex code so some extra effort to give any new readers an overview of how things work seems like a good idea. Author: Amit Langote Reported-by: Andy Fan Reviewed-by: Kyotaro Horiguchi, Andy Fan, Ryan Lambert, David Rowley Discussion: https://postgr.es/m/CAKU4AWqWoVii+bRTeBQmeVW+PznkdO8DfbwqNsu9Gj4ubt9A6w@mail.gmail.com
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