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b45d5368b0
I noticed that the timeline directory contained files like this:
pg_xact_0000_0_000000000169C3C2_00000000016BB399
pg_xact_0000_0_00000000016BB399
pg_xact_0000_0_00000000016BB399_00000000016BDD06
pg_xact_0000_0_00000000016BDD06
pg_xact_0000_0_00000000016BDD06_00000000016C63AA
pg_xact_0000_0_00000000016C63AA
pg_xact_0000_0_00000000016C63AA_0000000001765226_DROPPED
pg_xact_0000_0_0000000001765226
pg_xact_0001_0_00000000016BB77E_00000000016BDD06
pg_xact_0001_0_00000000016BDD06
pg_xact_0001_0_00000000016BDD06_0000000001765226_DROPPED
pg_xact_0001_0_0000000001765226
Note how there is an image file after each DROPPED file. It's a waste of
time and space to materialize an image of the file at the point where it's
dropped, no one is going to request pages on a dropped relation. And it's
a correctness issue too: list_rels() and list_nonrels() will not consider
the relation as unlinked, unless the latest layer indicates so, and there
is no concept of a dropped image layer. That was causing test_clog_truncate
test to fail, when I adjusted the checkpointer to force a checkpoint more
aggressively.
There are a bunch more issues related to dropped rels and branching,
see https://github.com/zenithdb/zenith/issues/502. Hence this doesn't
completely fix the issue I saw with test_clog_truncate either. But it's
a start.