English: a human brain in a jar
In-memory DBMS in a jar (Photo credit: Wikipedia)

I recently listened to a session by Juan Loalza of Oracle on the 12c In-memory option. Here are my notes and comments in order…

There is only one copy of the data on disk and that is in row format (“the column format does not exist on disk”). This has huge implications. Many of the implications come up later in the presentation… but consider: on start-up or recovery the data has to be loaded from the row format and converted to a columnar format. This is a very expensive undertaking.

The in-memory columnar representation is a fully redundant copy of the row format OLTP representation. Note that this should not impact performance as the transaction is gated by the time to write to the log and we assume that the columnar tables are managed by MVCC just like the row tables. It would be nice to confirm this.

Data is not as compressed in-memory as in the hybrid-columnar-compression case. This is explained in my discussion of columnar compression here.

Oracle claims that an in-memory columnar implementation (level 3 maturity by my measure see here) is up to 100X faster than the same implementation in a row-based form. Funny, they did not say that the week before OOW? This means, of course, that HANA, xVelocity, BLU, etc. are 100X faster today.

They had a funny slide talking about “reporting” but explained that this is another word for aggregation. Of course in-memory vector aggregation is faster in-memory.

There was a very interesting discussion of Oracle ERP applications. The speaker suggested that there is no reporting schema for these apps and that users therefore place indexes on the OLTP database to provide performance for reporting and analytics. It was suggested that a typical Oracle E-Business table would have 10-20 indexes on it and it was not unusual to see 30-40 indexes. It was even mentioned that the Siebel application main table could require 90 indexes. It was suggested that by removing these indexes you could significantly speed up the OLTP performance, speed up reporting and analytics, cure cancer and end all wars (OK… they did not suggest that you could cure cancer and end war… this post was just getting a little dry).

The In-memory option is clusterable. Further, because a RAC cluster uses shared disk and the im-memory data is not written to disk there is a new shared-nothing implementation included. This is a very nice and significant architectural advance for Oracle. It uses the direct-to-wire infiniband protocol developed for Exadata to exchange data. Remember when Larry dissed Teradata and shared-nothingness… remember when Larry dissed in-memory and HANA… remember when Teradata dissed HANA as nonsense and said SAP was over-reacting to Oracle. Gotta smile :).

Admins need to reserve memory from the SGA for the in-memory option. This is problematic for Exadata as the maximum memory on a RAC node is 256GB…  My bad… Exadata X3-8 supports 2TB of memory per node… this is only problematic for Exadata X3-2 and below… – Rob

It is possible to configure a table partition into memory while leaving other partitions in row format on disk.

It is suggested again that analytic indexes may be dropped to speed up OLTP.

The presenter talked about how the architecture, which does not change the row-based tables in any way, allows all of the high-availability options available with Oracle to continue to exist and operate unchanged.

But the beautiful picture starts to dull a little here. On start-up or first access… i.e. during recovery… the in-memory columnar data is unavailable and loaded asynchronously from the row format on disk. Note that it is possible to prioritize the order tables are loaded to get high-priority data in-memory first… a nice feature. During this time… which may be significant… all analytic queries are run against the un-indexed row store. Yikes! This kills OLTP performance and destroys analytic performance. In fact, the presenter suggested that maybe you might keep some indexes for reports on your OLTP tables to mitigate this.

Now the story really starts to unravel. Indexes are required to provide performance during recovery… so if your recovery SLA’s cannot be met without indexes then you are back to square one with indexes that are only used during recovery but must be maintained always, slower OLTP, and the extra requirement for a redundant in-memory columnar data image. I imagine that you could throttle some reports after an outage until the in-memory image is rebuilt… but the seamless operations during recovery using standard Oracle HA products is a bit of a stretch.

Let me raise again the question I asked in my post last week on this subject (here)… how are joins processed between the row format and the columnar format. The presenter says that joins are no problem… but there are really only two ways… maybe three ways to solve for this:

  1. When a row-to-columnar join is identified the optimizer converts the columnar table to a row form and processes the join using the row engine. This would be very very slow as there would be no indexes on the newly converted columnar data to facilitate the join.
  2. When a row-to-columnar join is identified the optimizer pushes down aggregation and projection to the columnar processing engine and converts the columnar result to a row form and processes the join using the row engine. This would be moderately slow as there would be no indexes on the newly converted columnar data to facilitate the join (i.e. the columnar fact would have no indexes to row dimensions or visa versa).
  3. When a row-to-columnar join is identified the optimizer converts the row table to a columnar form and processes the join using the columnar engine. This could be fast.

Numbers two and three are the coolest options… but number three is very unlikely due to the fact that columnar data is sharded in a shared-nothing manner and the row data is not… and number two is unlikely because if it were the case Oracle would surely be talking about it. Number one is easy and the likely state in release 1… but these joins will be very slow.

Finally, the presenter said that this columnar processing would be implemented in Times Ten and then in the Exalytics machine. I do not really get the logic here? If a user can aggregate in their OLTP system in a flash why would they pre-aggregate data and pass it to another data stovepipe? If you had to offload workload from your OLTP system why wouldn’t you deploy a small, standard, Oracle server with the in-memory option and move data there where, as the presenter suggested, you can solve any query fast… not just the pre-aggregated queries served by Exalytics. Frankly, I’ve wondered why SAP has not marketed a small HANA server as an Exalytics replacement for just this reason… more speed… more agility… same cost?

There you have it… my half a cents… some may say cents-less evaluation.

I will end this with a question for my audience (Ofir… I’ll provide a link to your site if you post on this…)… how do we suppose the in-memory option supports bulk data load? This has implications for data warehousing…

Here, of course, is the picture I should have used above… labeled as the in-memory database of your favorite vendor:

abbyn