Hi hackers, I noticed that the comments regarding bit layouts for varlena headers in postgres.h are somewhat misleading. For instance, when reading:
``` 00xxxxxx 4-byte length word, aligned, uncompressed data (up to 1G) ``` ... one can assume this is a 00xxxxxx byte followed by another 4 bytes (which is wrong). Also one can read this as "aligned, uncompressed data" (which again is wrong). ``` 10000000 1-byte length word, unaligned, TOAST pointer ``` This is misleading too. The comments above this line say that `struct varatt_external` is a TOAST pointer. sizeof(varatt_external) = 16, plus 1 byte equals 17, right? However the documentation [1] claims the result should be 18: """ Allowing for the varlena header bytes, the total size of an on-disk TOAST pointer datum is therefore 18 bytes regardless of the actual size of the represented value. """ I did my best to get rid of any ambiguity. The patch is attached. [1]: https://www.postgresql.org/docs/current/storage-toast.html -- Best regards, Aleksander Alekseev
v1-0001-Clarify-the-comments-about-varlena-header-encodin.patch
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