21b446dd09
In commit 7b0d0e9356963d5c3e4d329a917f5fbb82a2ef05, I made CLUSTER and VACUUM FULL try to preserve toast value OIDs from the original toast table to the new one. However, if we have to copy both live and recently-dead versions of a row that has a toasted column, those versions may well reference the same toast value with the same OID. The patch then led to duplicate-key failures as we tried to insert the toast value twice with the same OID. (The previous behavior was not very desirable either, since it would have silently inserted the same value twice with different OIDs. That wastes space, but what's worse is that the toast values inserted for already-dead heap rows would not be reclaimed by subsequent ordinary VACUUMs, since they go into the new toast table marked live not deleted.) To fix, check if the copied OID already exists in the new toast table, and if so, assume that it stores the desired value. This is reasonably safe since the only case where we will copy an OID from a previous toast pointer is when toast_insert_or_update was given that toast pointer and so we just pulled the data from the old table; if we got two different values that way then we have big problems anyway. We do have to assume that no other backend is inserting items into the new toast table concurrently, but that's surely safe for CLUSTER and VACUUM FULL. Per bug #6393 from Maxim Boguk. Back-patch to 9.0, same as the previous patch.
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