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bde7493d10
The comment did not match what the code actually did for integers with the 43rd bit set. You get an integer like that, if you have a posting list with two adjacent TIDs that are more than 2^31 blocks apart. According to the comment, we would store that in 6 bytes, with no continuation bit on the 6th byte, but in reality, the code encodes it using 7 bytes, with a continuation bit on the 6th byte as normal. The decoding routine also handled these 7-byte integers correctly, except for an overflow check that assumed that one integer needs at most 6 bytes. Fix the overflow check, and fix the comment to match what the code actually does. Also fix the comment that claimed that there are 17 unused bits in the 64-bit representation of an item pointer. In reality, there are 64-32-11=21. Fitting any item pointer into max 6 bytes was an important property when this was written, because in the old pre-9.4 format, item pointers were stored as plain arrays, with 6 bytes for every item pointer. The maximum of 6 bytes per integer in the new format guaranteed that we could convert any page from the old format to the new format after upgrade, so that the new format was never larger than the old format. But we hardly need to worry about that anymore, and running into that problem during upgrade, where an item pointer is expanded from 6 to 7 bytes such that the data doesn't fit on a page anymore, is implausible in practice anyway. Backpatch to all supported versions. This also includes a little test module to test these large distances between item pointers, without requiring a 16 TB table. It is not backpatched, I'm including it more for the benefit of future development of new posting list formats. Discussion: https://www.postgresql.org/message-id/33bfc20a-5c86-f50c-f5a5-58e9925d05ff%40iki.fi Reviewed-by: Masahiko Sawada, Alexander Korotkov |
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