The temporal feature works for both on-disk and memory-optimized tables, but has a slightly different implementation for memory-optimized tables.
As changes are made to rows in the temporal memory-optimized table, before being transferred to the history table on disk, they are first migrated to an internal memory-optimized staging table. That means when you query the “history table”, you could be retrieving rows from both the on-disk history table, and internal staging table. Because no custom indexing was possible on the internal staging table, there could be performance implications when executing queries against historical data. Microsoft addressed these potential performance issues in SQL 2016 SP1 (detailed in this CAT blog post).
The internal staging table only gets flushed to the on-disk history table when it reaches 8% of the size of the temporal table. Given the current capacities of Windows Server 2016 (24TB memory), it’s now possible to have very large memory-optimized tables. 8% of one of those large memory-optimized tables could be quite large, which will affect query performance, if predicates don’t match available indexes.
As of SP1 you can address the performance issues by adding (multiple) indexes to the internal staging table, and while that’s a fantastic improvement, there are some things to be aware of:
- The fix requires Trace Flag 10316, so it’s one more thing you’ll have to remember to configure in your environments.
- The name of the internal staging table is dynamic. This means that the script you maintain to recreate indexes on the internal table must first determine the name of the internal staging table (the CAT post details how to do this). And you’ll have the same issue for failover, whether it’s FCI or AG.
Now imagine you have dozens (or multiple dozens) of memory-optimized tables that use the temporal feature, and you want to customize indexing differently on all of them. The current SP1 solution doesn’t seem like such a great fix when DBAs must maintain dozens of scripts to apply custom indexing upon server reboot or failover.
There is perhaps a simpler solution, and that would be to monitor the number of rows in the internal staging table, and flush it to disk once it hits a threshold. Don’t assume that you’ll want to flush at the same threshold for all tables. And this won’t solve the custom indexing issue, but it could make queries perform less-worse until they’re flushed to the history table, where presumably you already have custom indexing in place. But again, you’ll have the dynamic table name issue.
You’d have to create a SQL Agent job that checks the rowcount for internal staging tables, and then call sys.sp_xtp_flush_temporal_history if required.
Your script would have to be intelligent enough to determine which tables are memory-optimized, and whether or not a given memory-optimized table uses the temporal feature.
And when you add/remove the temporal feature for a memory-optimized table, you’ll have to remember to update the custom script and Agent job. And of course this custom script will have to be executed upon reboot and/or failover.
This is just one more thing to be aware of when you consider deploying the temporal feature with In-Memory OLTP.