We have been using Postgres 9.5.12 behind PGBouncer and facing some weird issues. Whenever we running long running DDL statements (e.g. 'add index concurently' or 'Alter table alter column type'), after some time, we start seeing that almost every query in our application starts getting blocked.
I understand that the operations I mentioned cab be unsafe, but the queries being blocked are on completely unrelated tables. I used the instructions given on Postgres wiki (https://wiki.postgresql.org/wiki/Lock_Monitoring) to correlate the blocking and blocked statements and there seems to be absolutely no correlation.