decoding – select * from depesz;

We had a case recently where one of our DBs failed over to a new primary. To this server (old primary, and then new primary) we had connection from some kind of CDC tool (Debezium, I think).

The thing is that while there was failover, this tool read (using logical decoding) changes on old primary to location 1F1F/4ADC3348, and on new db, it connected from location 1F1F/4ADC4038. Which means that there was some WAL that was not decoded and sent to Debezium.

This is fixable, but we need to know what tables were touched in the WAL stream, that Debezium didn't see, if any. Perhaps we could also get ids/location of the rows that were there?

Continue reading What tables were touched within given range of wal LSN?

On 3rd of March, Robert Haas committed patch:

Introduce logical decoding.
 
This feature, building on previous commits, allows the write-ahead log
stream to be decoded into a series of logical changes; that is,
inserts, updates, and deletes and the transactions which contain them.
It is capable of handling decoding even across changes to the schema
of the effected tables.  The output format is controlled by a
so-called "output plugin"; an example is included.  To make use of
this in a real replication system, the output plugin will need to be
modified to produce output in the format appropriate to that system,
and to perform filtering.
 
Currently, information can be extracted from the logical decoding
system only via SQL; future commits will add the ability to stream
changes via walsender.
 
Andres Freund, with review and other contributions from many other
people, including Álvaro Herrera, Abhijit Menon-Sen, Peter Gheogegan,
Kevin Grittner, Robert Haas, Heikki Linnakangas, Fujii Masao, Abhijit
Menon-Sen, Michael Paquier, Simon Riggs, Craig Ringer, and Steve
Singer.

Continue reading Waiting for 9.4 – Introduce logical decoding

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Tag: decoding

What tables were touched within given range of wal LSN?

Waiting for 9.4 – Introduce logical decoding