Re: [HACKERS] Custom compression methods (mac+lz4.h)

Dilip Kumar <dilipbalaut(at)gmail(dot)com> writes:
>> Yeah, we need to set the default_toast_compression in the beginning of
>> the test as attached.
> In the last patch, I did not adjust the compression_1.out so fixed
> that in the attached patch.

Pushed that; however, while testing that it works as expected,
I saw a new and far more concerning regression diff:

diff -U3 /home/postgres/pgsql/src/test/regress/expected/strings.out /home/postgres/pgsql/src/test/regress/results/strings.out
--- /home/postgres/pgsql/src/test/regress/expected/strings.out 2021-02-18 10:34:58.190304138 -0500
+++ /home/postgres/pgsql/src/test/regress/results/strings.out 2021-03-21 16:27:22.029402834 -0400
@@ -1443,10 +1443,10 @@
-- If start plus length is > string length, the result is truncated to
-- string length
SELECT substr(f1, 99995, 10) from toasttest;
- substr
---------
- 567890
- 567890
+ substr
+------------------------
+ 567890\x7F\x7F\x7F\x7F
+ 567890\x7F\x7F\x7F\x7F
567890
567890
(4 rows)
@@ -1520,10 +1520,10 @@
-- If start plus length is > string length, the result is truncated to
-- string length
SELECT substr(f1, 99995, 10) from toasttest;
- substr
---------
- 567890
- 567890
+ substr
+------------------------
+ 567890\177\177\177\177
+ 567890\177\177\177\177
567890
567890
(4 rows)

This seems somewhat repeatable (three identical failures in three
attempts). Not sure why I did not see it yesterday; but anyway,
there is something wrong with partial detoasting for LZ4.

regards, tom lane