On 10/30/16 9:12 AM, Tom Lane wrote:
I think there will be a lot of howls. People expect that creating a table by inserting a bunch of rows, and then reading back those rows, will not change the order. We already futzed with that guarantee a bit with syncscans, but that only affects quite large tables --- and even there, we were forced to provide a way to turn it off.
Leaving a 30% performance improvement on the floor because some people don't grok how sets work seems insane to me.
We could have a GUC to disable this. I suspect ORDER BY ctid would be another option.
BTW, I've sometimes wished for a mode where queries would silently have result ordering intentionally futzed, to eliminate any possibility of dependence on tuple ordering (as well as having sequences start at some random value). I guess with the hooks that are in place today it wouldn't be hard to stick a ORDER BY random() in if there wasn't already a Sort node at the top level...
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