On Wed, Sep 1, 2021, at 9:36 AM, Amit Kapila wrote:
I think this or any other similar solution for row filters (on
updates) won't work till we solve the problem reported by Hou-San [1].
The main reason is that we don't have data for unchanged toast columns
in WAL. For that, we have discussed some probable solutions in email
[2], however, that also required us to solve one of the existing
bugs[3].
I didn't mention but I'm working on it in parallel.
I agree with you that including TOAST values in the WAL is a possible solution
for this issue. This is a popular request for wal2json [1][2][3] and I think
other output plugins have the same request too. It is useful for CDC solutions.
I'm experimenting 2 approaches: (i) always include unchanged TOAST values to
new tuple if a GUC is set and (ii) include unchanged TOAST values to new tuple
iif it wasn't include in the old tuple. The advantage of the first option is
that you fix the problem adjusting a parameter in your configuration file.
However, the disadvantage is that, depending on your setup -- REPLICA IDENTITY
FULL, you might have the same TOAST value for a single change twice in the WAL.
The second option solves the disadvantage of (i) but it only works if you have
REPLICA IDENTITY FULL and Dilip's patch applied [4] (I expect to review it
soon). In the output plugin, (i) requires a simple modification (remove
restriction for unchanged TOAST values) but (ii) needs a more complex surgery.