Re: Optimize partial TOAST decompression

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)

Before below commit:

commit 540f31680913b4e11f2caa40cafeca269cfcb22f
Author: Tomas Vondra <tomas.vondra@postgresql.org>
Date:   Tue Oct 1 16:53:04 2019 +0200

    Blind attempt to fix pglz_maximum_compressed_size
   
    Commit 11a078cf87 triggered failures on big-endian machines, and the
    only plausible place for an issue seems to be that TOAST_COMPRESS_SIZE
    calls VARSIZE instead of VARSIZE_ANY. So try fixing that blindly.
   
    Discussion: https://www.postgresql.org/message-id/20191001131803.j6uin7nho7t6vxzy%40development

postgres@75761=#select substring(a from 639921 for 81) from foo;

                                    substring                                    
----------------------------------------------------------------------------------
 PostgreSQL is the world's best database and leading by an Open Source Community.
(1 row)