Re: Vacuum not cleaning up rows.

[SBob <sbob@xxxxxxxxxxxxxxxxxxxxx>]

Got it. can someone point me to more reading on snapshots  and snapshot-horizon?

Thanks!

On Fri, Jun 21, 2019, at 10:08 AM, Tom Lane wrote:

SBob <sbob@xxxxxxxxxxxxxxxxxxxxx> writes:

> I do see older backend_start values (from around 6 hours ago). However I am confused as to how a connection to one database in a cluster can prevenr vacuum from cleaning up row versions in a table within another database. How is that possible since onle one database has access to this table?

Because the snapshot-horizon calculations are done system-wide.

We can't just lobotomize them to only consider the current DB,

because that would result in wrong row-cleanup decisions for shared

catalogs.

There are some attempts to refine that when vacuuming a non-shared table,

but that can only go so far given the limited amount of state that

backends expose about this.  Doing better would require expending

more effort in transaction start/transaction stop/snapshot creation,

which are hot spots so it hasn't seemed like it'd be a good tradeoff.

regards, tom lane