Thread: PITR on DROP DATABASE, deleting of the database directory despite therecovery_target_time set before.

On 11/19/19 1:40 AM, Craig Ringer wrote:
> On Mon, 18 Nov 2019 at 18:48, Nicolas Lutic <n.lutic@loxodata.com
> <mailto:n.lutic@loxodata.com>> wrote:
> 
>     Dear Hackers,
> 
>     After a drop database
> 
> 
> with FORCE?
No, we tested with PostgreSQL v 11 and we don't have this option.
>  
> 
>     , he tried to recover the data on the last inserted transaction by
>     using the recovery_target_time.
>     The issue is the database is present in the system catalog but the
>     directory was still deleted.
>     Here the technical information of the database
>     version 11
>     default  postgresql.conf except for this options
>         wal_level = replica
>         archive_mode = on
>         archive_command = 'cp %p /tmp/wal_archive/%f '
>         log_statement = 'all'
>         log_min_messages = debug5
> 
>       
>     The following method was used 
> 
>       * create cluster
> 
>       * create database
> 
>       * create 1 table 
> 
>       * create 1 index on 1 column
> 
>       * insert 1 rows
> 
>       * backup with pg_base_backup
> 
>       * insert 2 rows
> 
> autocommit? 

Yes, I forgot to mention it.

> 
>       * drop database
> 
> force?
>  
> 
>       *     Change recovery behaviour in that case to prevent all xact
>         operation to perform until COMMIT timestamp is checked against
>         recovery_time bound (but it seems to be difficult as
>         state https://www.postgresql.org/message-id/flat/20141125160629.GC21475%40msg.df7cb.dewhich
>         also identifies the problem and tries to give some solutions. 
>         Maybe another way, as a trivial guess (all apologises) is to
>         buffer immediate xacts until we have the commit for each and
>         apply the whole buffer xact once the timestamp known (and
>         checked agains recovery_target_time value);
> 
>       *     The other way to improve this is to update PostgreSQL
>         documentation  by specifying that recovery_target_time cannot be
>         used in this case.There should be multiple places where it can
>         be stated. The best one (if only one) seems to be in 
>         https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=doc/src/sgml/config.sgml;h=
>
<https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=doc/src/sgml/config.sgml;h=f83770350eda5625179526300c652f23ff29c9fe;hb=HEAD#l3400>
> 
> 
> If this only happens when a DB is dropped under load with force, I lean
> toward just documenting it as a corner case.

This can happen in the case of a non-transactional instruction, DROP
DATABASE (with or without FORCE) is one case but there may be other cases ?

The documentation modification have to mention this case and list the
other most likely operations.

An idea, without insight knowledge of the code, in case of
recovery_target_time (only), would be to move forward each record for an
xact.

Each record that is «timestamped» can be applied but once we encounter a
non timestamped record we could buffer the following records for any
xaxts until a timestamped commit/rollback for the transaction where that
non transactionnal op appearsin. Once the commit/rollback records are
found, there's two options :
    1) the commit/rollback timestamp is inside the "replay" bound, then the
whole buffer can be applied
    2) the commit/rollback timestamp is beyond the upper time bound for
"replay", then  the whole buffer for that transaction could be canceled.
This can only be done on DROP DATABASE "DELETE" operation ?
Maybe, this will lead to skewed pages and this is a wrong way to do such
a thing.

Another assumption is that "DROP DATABASE" sequence can be changed for
this operation to perform correctly.

We are aware that this part is tricky and will have little effects on
normal operations, as best practices are to use xid_target or lsn_target.


> 
> -- 
>  Craig Ringer                   http://www.2ndQuadrant.com/
>  2ndQuadrant - PostgreSQL Solutions for the Enterprise

Best regards

-- 
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