Re: Why could different data in a table be processed with different performance?

FYI, posting an intermediate update on the issue.

I disabled index scans to keep existing order, and copied part of the
"slow" range into another table (3M rows in 2.2 GB table + 17 GB toast). I
was able to reproduce slow readings from this copy. Then I performed
CLUSTER of the copy using PK and everything improved significantly. Overall
time became 6 times faster with disk read speed (reported by iotop)
30-60MB/s.

I think we can take bad physical data distribution as the main hypothesis
of the issue. I was not able to launch seekwatcher though (it does not work
out of the box in Ubuntu and I failed to rebuild it) and confirm lots of
seeks.

I still don't have enough disk space to solve the problem with original
table, I am waiting for this from admin/devops team.

My plan is to partition the original table and CLUSTER every partition on
primary key once I have space.

Best regards,
Vlad