Here's my test results so far with the github archive data.
It's important to keep in mind that the PushEvent event objects that I use in the queries only contains a small number of keys (8 to be precise), so these tests don't really stress the changed code.
Anyway, in this dataset (with the small objects) using the all-lengths patch provide small compression savings but the overhead is minimal.
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Test data: 610MB of Json -- 341969 items
Index size (jsonb_ops): 331MB
Test query 1: SELECT data->'url', data->'actor' FROM t_json WHERE data @> '{"type": "PushEvent"}' Test query 1 items: 169732
Test query 2: SELECT data FROM t_json WHERE data @> '{"type": "PushEvent"}' Test query 2 items:
---------------- HEAD (aka, all offsets) EXTENDED Size: 374MB Toast Size: 145MB
Test query 1 runtime: 680ms Test query 2 runtime: 405ms ---------------- HEAD (aka, all offsets) EXTERNAL Size: 366MB Toast Size: 333MB
Test query 1 runtime: 505ms Test query 2 runtime: 350ms ---------------- All Lengths (Tom Lane patch) EXTENDED Size: 379MB Toast Size: 108MB
Test query 1 runtime: 720ms Test query 2 runtime: 420ms ---------------- All Lengths (Tom Lane patch) EXTERNAL Size: 366MB Toast Size: 333MB
Test query 1 runtime: 525ms Test query 2 runtime: 355ms
