Thread: Serializable read only deferrable- implications
https://www.postgresql.org/docs/14/sql-set-transaction.html
"The
DEFERRABLE transaction property has no effect unless the transaction is also SERIALIZABLE and READ ONLY. When all three of these properties are selected for a transaction, the transaction may block when first acquiring its snapshot, after which it is able to run without the normal overhead of a SERIALIZABLE transaction and without any risk of contributing to or being canceled by a serialization failure. This mode is well suited for long-running reports or backups."Could anyone expound on the above? What are the real life differences between the two commands below? Would this be applicable at all to sessions on a physical replica?
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE;
SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
Michael Lewis | Database Engineer
Entrata
On Tue, 2022-03-08 at 06:29 -0700, Michael Lewis wrote: > "The DEFERRABLE transaction property has no effect unless the transaction is also > SERIALIZABLE and READ ONLY. When all three of these properties are selected for a transaction, > the transaction may block when first acquiring its snapshot, after which it is able to run > without the normal overhead of a SERIALIZABLE transaction and without any risk of contributing > to or being canceled by a serialization failure. This mode is well suited for long-running reports or backups." > > > Could anyone expound on the above? What are the real life differences between the two commands below? Would this be applicableat all to sessions on a physical replica? > > SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE READ ONLY DEFERRABLE; > > SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY; A transaction started with the first statement will not take any SI locks, nor will it ever receive a serialization error. Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
A transaction started with the first statement will not take any SI locks, nor
will it ever receive a serialization error.
What is the meaning of SI? Anything you are aware of in source code or a blog post that discusses this?
Michael Lewis <mlewis@entrata.com> writes:
>> A transaction started with the first statement will not take any SI locks,
>> nor will it ever receive a serialization error.
> What is the meaning of SI? Anything you are aware of in source code or a
> blog post that discusses this?
There's src/backend/storage/lmgr/README-SSI, which might or might
not be the level of detail you are looking for.
regards, tom lane
On 3/8/22 08:21, Tom Lane wrote: > Michael Lewis <mlewis@entrata.com> writes: >>> A transaction started with the first statement will not take any SI locks, >>> nor will it ever receive a serialization error. > >> What is the meaning of SI? Anything you are aware of in source code or a >> blog post that discusses this? > > There's src/backend/storage/lmgr/README-SSI, which might or might > not be the level of detail you are looking for. If you want to ease into the above: https://www.postgresql.org/docs/current/mvcc-intro.html "PostgreSQL maintains this guarantee even when providing the strictest level of transaction isolation through the use of an innovative Serializable Snapshot Isolation (SSI) level." Then: https://www.postgresql.org/docs/current/transaction-iso.html#XACT-SERIALIZABLE and https://www.postgresql.org/docs/current/applevel-consistency.html#SERIALIZABLE-CONSISTENCY > > regards, tom lane > > -- Adrian Klaver adrian.klaver@aklaver.com
On Tue, Mar 8, 2022 at 9:27 AM Adrian Klaver <adrian.klaver@aklaver.com> wrote:
"PostgreSQL maintains this guarantee even when providing the strictest
level of transaction isolation through the use of an innovative
Serializable Snapshot Isolation (SSI) level."
Then:
https://www.postgresql.org/docs/current/transaction-iso.html#XACT-SERIALIZABLE
and
https://www.postgresql.org/docs/current/applevel-consistency.html#SERIALIZABLE-CONSISTENCY
The documentation says that a transaction ID is only assigned to a connection once a write is done, but is the assignment or not of a txn id actually impactful on anything? I ask partly because it doesn't seem possible to reset that once assigned, through discard all; or something else like that which might be used by a connection pooler such as pg bouncer. is there any way to check if a session has "done writes/updates up to this point"? It seems pg_my_temp_schema() also returns the same value even after 'discard temp' or 'discard all' is executed. That was surprising to me, but would it be considered an issue by anyone?
On 3/8/22 10:47 AM, Michael Lewis wrote: > Thanks to you both. If other concurrent sessions are using default > isolation level of Read committed, would putting long running reports > (read-only) into that read-only serializable deferrable mode be > impactful at all? > > The documentation says that a transaction ID is only assigned to a > connection once a write is done, but is the assignment or not of a txn > id actually impactful on anything? I ask partly because it doesn't seem > possible to reset that once assigned, through discard all; or something > else like that which might be used by a connection pooler such as pg > bouncer. is there any way to check if a session has "done writes/updates > up to this point"? It seems pg_my_temp_schema() also returns the same > value even after 'discard temp' or 'discard all' is executed. That was > surprising to me, but would it be considered an issue by anyone? I'm not following what you are asking or trying to achieve. For instance how pg_my_temp_schema() fits into this? You will need to provide a more complete description of what it is you are doing. -- Adrian Klaver adrian.klaver@aklaver.com
On Tue, Mar 8, 2022 at 11:47 AM Michael Lewis <mlewis@entrata.com> wrote:
Thanks to you both. If other concurrent sessions are using default isolation level of Read committed, would putting long running reports (read-only) into that read-only serializable deferrable mode be impactful at all?
I'm tending to agree that this documentation could be improved (I too am not totally fluent on how the different modes interact with each other). In this specific case the docs say:
"When all three of these properties are selected for a transaction, the transaction may block when first acquiring its snapshot,..."
They fail to say what they are blocking on and thus what has to occur in the system before the lock can be acquired and the transaction continues onward. I suspect it is simply that any not yet committed transactions become committed or reverted. Whether those transactions were run under serializable, read committed, or repeatable read would be immaterial.
The documentation says that a transaction ID is only assigned to a connection once a write is done, but is the assignment or not of a txn id actually impactful on anything?
I ask partly because it doesn't seem possible to reset that once assigned, through discard all; or something else like that which might be used by a connection pooler such as pg bouncer. is there any way to check if a session has "done writes/updates up to this point"? It seems pg_my_temp_schema() also returns the same value even after 'discard temp' or 'discard all' is executed. That was surprising to me, but would it be considered an issue by anyone?
I'm getting an impression that you have confusion regarding transactions and sessions, since all of this concurrency stuff only cares about the transactions but you now start talking about session state and comparing what you would see at initial login versus what you see after resetting.
The temporary schema for a session is assigned once the first temporary object is created. At which point that assignment of the random temporary schema is fixed for the duration of the session. It never gets reset as there is no point.
Whether a session has done "write/updates up to this point" is likewise not something that can be readily ascertained nor, from what I can tell, would doing so be a useful exercise. There may be some reason to inspect whether a transaction has performed a write/update, but even that seems unlikely.
David J.
Sorry for the confusion I caused. The question about connection management and pg bouncer was a distraction and should have been addressed separately.
When having a mixture of OLTP and OLAP on the same primary databases, is there any benefit to declaring long running report type connections as SERIALIZABLE READ ONLY DEFERRABLE in terms of impact on logical or physical replication, autovacuum, etc even if the much heavier OLTP traffic is still running as the default read committed mode?
If the OLAP queries are moved to a physical read replica, I understand from this post ( https://www.cybertec-postgresql.com/en/streaming-replication-conflicts-in-postgresql/ ) that there are chances that a query will be killed on the replica even with hot_standby_feedback is turned on. With them running on the same server, is the main concern (other than load) that vacuum type cleanup is delayed?
Maybe to sum up- If a long running report type query is run in default "read committed" mode and uses no temp tables / does no writes, would there be a benefit or change in behavior when using SERIALIZABLE READ ONLY DEFERRABLE mode?
When having a mixture of OLTP and OLAP on the same primary databases, is there any benefit to declaring long running report type connections as SERIALIZABLE READ ONLY DEFERRABLE in terms of impact on logical or physical replication, autovacuum, etc even if the much heavier OLTP traffic is still running as the default read committed mode?
If the OLAP queries are moved to a physical read replica, I understand from this post ( https://www.cybertec-postgresql.com/en/streaming-replication-conflicts-in-postgresql/ ) that there are chances that a query will be killed on the replica even with hot_standby_feedback is turned on. With them running on the same server, is the main concern (other than load) that vacuum type cleanup is delayed?
Maybe to sum up- If a long running report type query is run in default "read committed" mode and uses no temp tables / does no writes, would there be a benefit or change in behavior when using SERIALIZABLE READ ONLY DEFERRABLE mode?