Re: Global temporary tables

On 20.08.2019 20:01, Pavel Stehule wrote:

Another solution is wait on ZHeap storage and replica can to have own UNDO log.

I thought about implementation of special table access method for temporary tables.

+1

 

Unfortunately implementing special table access method for temporary tables doesn't solve all problems.
XID generation is not part of table access methods.
So we still need to assign some XID to write transaction at replica which will not conflict with XIDs received from master.
Actually only global temp tables can be updated at replica and so assigned XIDs can be stored only in tuples of such relations.
But still I am not sure that we can use arbitrary XID for such transactions at replica.

Also I upset by amount of functionality which has to be reimplemented for global temp tables if we really want to provide access method for them:

1. CLOG
2. vacuum
3. MVCC visibility

And still it is not possible to encapsulate all changes need to support writes to temp tables at replica inside table access method.
XID assignment, transaction commit and abort, subtransactions - all this places need to be patched.

I was able to fully support work with global temp tables at replica (including subtransactions).
The patch is attached. Also you can find this version in https://github.com/postgrespro/postgresql.builtin_pool/tree/global_temp_hot

Right now transactions at replica updating global temp table are assigned special kind of GIDs which are not related with XIDs received from master.
So special visibility rules are used for such tables at replica. Also I have to patch TransactionIdIsInProgress, TransactionIdDidCommit, TransactionIdGetCurrent
functions to correctly handle such XIDs. In principle it is possible to implement global temp tables as special heap access method. But it will require copying a lot of code (heapam.c)
so I prefer to add few checks to existed functions.

There are still some limitations:
- Number of transactions at replica which update temp tables is limited by 2^32 (wraparound problem is not addressed).
- I have to maintain in-memory analog of CLOG for such transactions which is also not cropped. It means that for 2^32 transaction size of bitmap can grow up to  0.5Gb.

I try to understand what are the following steps in global temp tables support.
This is why I want to perform short survey - what people are expecting from global temp tables:

1. I do not need them at all.
2. Eliminate catalog bloating.
3. Mostly needed for compatibility with Oracle (simplify porting,...).
4. Parallel query execution.
5. Can be used at replica.
6. More efficient use of resources (first of all memory).

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company