On Thu, Nov 14, 2019 at 03:27:42PM +0530, Rushabh Lathia wrote:
Today I noticed strange behaviour, consider the following test:
postgres@126111=#create table foo ( a text );
CREATE TABLE
postgres@126111=#insert into foo values ( repeat('PostgreSQL is the
world''s best database and leading by an Open Source Community.', 8000));
INSERT 0 1
postgres@126111=#select substring(a from 639921 for 81) from foo;
substring
-----------
(1 row)
Hmmm. I think the issue is heap_tuple_untoast_attr_slice is using the
wrong way to determine compressed size in the VARATT_IS_EXTERNAL_ONDISK
branch. It does this
max_size = pglz_maximum_compressed_size(sliceoffset + slicelength,
TOAST_COMPRESS_SIZE(attr));
But for the example you've posted TOAST_COMPRESS_SIZE(attr) returns 10,
which is obviously bogus because the TOAST table contains ~75kB of data.
I think it should be doing this instead:
max_size = pglz_maximum_compressed_size(sliceoffset + slicelength,
toast_pointer.va_extsize);
At least that fixes it for me.
I wonder if this actually explains the crashes 540f3168091 was supposed
to fix, but it just masked them instead.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
diff --git a/src/backend/access/common/detoast.c
b/src/backend/access/common/detoast.c
index 47a03fa98b..b796406239 100644
--- a/src/backend/access/common/detoast.c
+++ b/src/backend/access/common/detoast.c
@@ -233,7 +233,7 @@ heap_tuple_untoast_attr_slice(struct varlena *attr,
* of a given length (after decompression).
*/
max_size = pglz_maximum_compressed_size(sliceoffset +
slicelength,
-
TOAST_COMPRESS_SIZE(attr));
+
toast_pointer.va_extsize);
/*
* Fetch enough compressed slices (compressed marker
will get set
