Re: Seems like there is an issue with reltuples showing twice the number of rows

List,

OP here. Thank you for replying. Confirms my diagnosis that it might have
to do with analyze vaccum.

Some debug info.

1. Loaded a CSV to fill the table with data.
2. performed analyse vacuum on this table after uploading.
3. I do not see any reason for dead rows because I have not updated data in
this table. But I may not understand dead rows correctly.
4. I can reproduce this problem on multiple machines with 9.6.8 postres
installed.

Can not wait for next minor update since my prod would get updated. My
current strategy is to use n_live_tup. On my local it seems to work fine.

Thinking about it, I could even develop another mechanism for keeping a
track of row counts by manually scanning the row count with a background
process, a hard count with count(*) too.

But happy provide debug any other info if needed. Will reply within 24
hours max.

This is what I had found earlier before I contacted the list. Relevant?

1. https://www.postgresql.org/message-id/20180312231417.484d64c0%40engels
2.
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=81b9b5ce490a645bde8df203ec4a3b2903d88f31
3.
https://www.postgresql.org/message-id/151956654251.6915.675951950408204404.pgcf@coridan.postgresql.org

On Tue 3 Apr, 2018, 19:49 Tom Lane, <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> > On 04/03/2018 11:14 AM, Ranjith Ramachandra wrote:
> >> it returns
> >> reltuples | n_live_tup | n_dead_tup
> >> -------------+------------+------------
> >> 2.7209e+06 | 1360448 | 1360448
> >>
> >> If I run analyze main_csv_0f247511f5c247c8813ba3cec90c6ae1_yf34fbb38d
> >> and I run the same query again,
> >> reltuples | n_live_tup | n_dead_tup
> >> -------------+------------+------------
> >> 1.36045e+06 | 1360448 | 1360448
> >>
> >> But after some time the value goes back to being double the value.
>
> > There was a difference between VACUUM and ANALYZE in handling recently
> > dead rows (essentially deleted rows that can't be removed yet), causing
> > similar changes to reltuples. Essentially if you do VACUUM and ANALYZE,
> > it may set reltuples to rather different estimates. That is fixed now
> > and should be in the next minor release.
>
> No, I think this is the *other* thing we fixed recently: VACUUM thinks
> it should set reltuples to total tuples (live + dead) whereas ANALYZE
> counts only live tuples. We did not risk back-patching that.
>
> The question I'd ask about this case is why is there persistently 100%
> bloat? Those dead tuples should've gotten reclaimed by autovacuum.
> Perhaps an open prepared transaction, or some such?
>
> > It's probably better to use n_live_tup instead, though. I'd say that's
> > closer to the "live tuples" definition.
>
> Yeah, you might be better off looking at that, particularly since it
> updates on-the-fly not just after a vacuum or analyze.
>
> regards, tom lane
>