postgres

Commit Graph

Author	SHA1	Message	Date
Alexander Korotkov	e85662df44	Fix false reports in pg_visibility Currently, pg_visibility computes its xid horizon using the GetOldestNonRemovableTransactionId(). The problem is that this horizon can sometimes go backward. That can lead to reporting false errors. In order to fix that, this commit implements a new function GetStrictOldestNonRemovableTransactionId(). This function computes the xid horizon, which would be guaranteed to be newer or equal to any xid horizon computed before. We have to do the following to achieve this. 1. Ignore processes xmin's, because they consider connection to other databases that were ignored before. 2. Ignore KnownAssignedXids, because they are not database-aware. At the same time, the primary could compute its horizons database-aware. 3. Ignore walsender xmin, because it could go backward if some replication connections don't use replication slots. As a result, we're using only currently running xids to compute the horizon. Surely these would significantly sacrifice accuracy. But we have to do so to avoid reporting false errors. Inspired by earlier patch by Daniel Shelepanov and the following discussion with Robert Haas and Tom Lane. Discussion: https://postgr.es/m/1649062270.289865713%40f403.i.mail.ru Reviewed-by: Alexander Lakhin, Dmitry Koval	2 years ago

Author

SHA1

Message

Date

Alexander Korotkov

e85662df44

Fix false reports in pg_visibility

Currently, pg_visibility computes its xid horizon using the
GetOldestNonRemovableTransactionId().  The problem is that this horizon can
sometimes go backward.  That can lead to reporting false errors.

In order to fix that, this commit implements a new function
GetStrictOldestNonRemovableTransactionId().  This function computes the xid
horizon, which would be guaranteed to be newer or equal to any xid horizon
computed before.

We have to do the following to achieve this.

1. Ignore processes xmin's, because they consider connection to other databases
   that were ignored before.
2. Ignore KnownAssignedXids, because they are not database-aware. At the same
   time, the primary could compute its horizons database-aware.
3. Ignore walsender xmin, because it could go backward if some replication
   connections don't use replication slots.

As a result, we're using only currently running xids to compute the horizon.
Surely these would significantly sacrifice accuracy.  But we have to do so to
avoid reporting false errors.

Inspired by earlier patch by Daniel Shelepanov and the following discussion
with Robert Haas and Tom Lane.

Discussion: https://postgr.es/m/1649062270.289865713%40f403.i.mail.ru
Reviewed-by: Alexander Lakhin, Dmitry Koval

2 years ago

1 Commits (d66a922f9247ceaa6caed9b51ce686f12e280091)