Shout out to team — we are seeing huge reduction in index sizes when upgrading to PG13 👏
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The question asked by is what version they upgraded *from*, which is a good question. If it was 11 or earlier then this must be effects of enhancements in both Postgres 12 and 13 (though still mostly the latter). Whereas 12 -> 13 means it's strictly dedup feature.
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Right...I ask because we recently upgraded to PG12, so just wanted to get a sense for what additional improvements we'd get moving to PG13 from the dedup feature
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It depends almost entirely on data cardinality. Whether or not it helps (and the extent to which it helps) varies by index. You're very likely to have at least one individual index that's 3x smaller, but no more than ~3.5x smaller. Back of an envelope estimates will work here.
In particular, the tables at the top of these results, like posts / post_timings / user_actions have a topic_id or post_id field that is indexed (INDEX posts ON topic_id) - a topic has many posts, and many users react to the exact same post - huge dedup wins
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