That's quite bizarre behavior - it does work with a prefix, but not with suffix. And the exact ERROR changes after the prefix query. (Of course, on master it works in all cases.)
The backtrace (with the patch applied) looks like this:
#0 toast_decompress_datum (attr=0x12572e0) at tuptoaster.c:2291 #1 toast_decompress_datum (attr=0x12572e0) at tuptoaster.c:2277 #2 0x00000000004c3b08 in heap_tuple_untoast_attr_slice (attr=<optimized out>, sliceoffset=0, slicelength=-1) at tuptoaster.c:315 #3 0x000000000085c1e5 in pg_detoast_datum_slice (datum=<optimized out>, first=<optimized out>, count=<optimized out>) at fmgr.c:1767 #4 0x0000000000833b7a in text_substring (str=133761519127512, start=0, length=<optimized out>, length_not_specified=<optimized out>) at varlena.c:956 ...
I've only observed this with a very small number of rows (the data is generated randomly with different compressibility etc.), so I'm only attaching one row that exhibits this issue.
My guess is toast_fetch_datum_slice() gets confused by the headers or something, or something like that. FWIW the new code added to this function does not adhere to our code style, and would deserve some additional explanation of what it's doing/why. Same for the heap_tuple_untoast_attr_slice, BTW.
regards
-- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Hi, Tomas!
Thanks for your testing and the suggestion.
That's quite bizarre behavior - it does work with a prefix, but not with suffix. And the exact ERROR changes after the prefix query.
I think bug is caused by "#2 0x00000000004c3b08 in heap_tuple_untoast_attr_slice (attr=<optimized out>, sliceoffset=0, slicelength=-1) at tuptoaster.c:315",
since I ignore the case where slicelength is negative, and I've appended some comments for heap_tuple_untoast_attr_slice for the case.
FWIW the new code added to this function does not adhere to our code style, and would deserve some additional explanation of what it's doing/why. Same for the heap_tuple_untoast_attr_slice, BTW.
I've added more comments to explain the code's behavior.
Besides, I also modified the macro "TOAST_COMPRESS_RAWDATA" to "TOAST_COMPRESS_DATA" since
it is used to get toast compressed data rather than raw data.