Re: Long text values destroys logical replication slots

On Thu, Oct 29, 2015 at 7:23 AM, Andres Freund <andres@anarazel.de> wrote:
> On 2015-10-28 23:00:53 +0100, Michael Paquier wrote:
>> On Wed, Oct 28, 2015 at 10:17 PM, Andres Freund <andres@anarazel.de> wrote:
>> > Can you reproduce it with test_decoding as the output plugin?
>>
>> You can just use that for example to get an assertion failure:
>> CREATE TABLE a (b text);
>> ALTER TABLE ONLY a REPLICA IDENTITY FULL;
>> SELECT * FROM pg_create_logical_replication_slot('new', 'test_decoding');
>> INSERT INTO a (b) VALUES (repeat('k', 2000000));
>> UPDATE a SET b = 'c';
>> select * from pg_logical_slot_peek_changes('new', NULL, NULL); -- boom
>>
>> frame #3: 0x0000000100458ca9
>> postgres`DecodeXLogTuple(data=0x00007fb7e2126046, len=22910,
>> tuple=0x000000010a32e038) + 137 at decode.c:856
>>    853         int            datalen = len - SizeOfHeapHeader;
>>    854
>>    855         Assert(datalen >= 0);
>> -> 856         Assert(datalen <= MaxHeapTupleSize);
>> (lldb) p datalen
>> (int) $0 = 22905
>
> Ugh, that's some monumental stupidity on my side in the handling of
> oldtuple values. Newtuple datums never can be bigger than 8k (all
> relevant columns point into toasted datums)- but that's absolutely not
> the case for the "old" values.  That can't happen for primary keys (due
> to the btree limitations), which is why this wasn't noticed so far.
> Gotta think about this one.

FWIW, I am going to play with that..
--
Michael