Tag: EDW

How DBMS Vendors Admit to an Architectural Limitation: Part 1 – Oracle Exadata

Database vendors don’t usually admit to shortcomings… they protest that they have no shortcomings until the market suggests otherwise… then they make some sort of change that signals an admission. This post will explore three of these admissions: Oracle and the shared-nothing architecture, DB2 on the mainframe and the shared-nothing architecture, and Teradata and in-memory processing.

For years Oracle verbally thrashed Teradata in the market… proclaiming that the shared-nothing architecture was bunk. But in the data warehousing space Teradata acquired a large chunk of the market; and more importantly, they won more business as the size of the data warehouses grew. The reason for this is two-fold: the shared-nothing architecture lets you deliver more I/O bandwidth to the problem… and once you have read the disk it provides scalability to deliver more compute to process complex queries.

Finally Oracle had enough and they delivered Exadata, a storage engine attached to the conventional Oracle RAC that provided shared-nothing I/O bandwidth to the biggest part of the problem… the full file scan of big fact tables. This was an admission that they had been wrong all along.

Exadata was a tack-on… not a fundamental redevelopment of the Oracle database engine. They used the 80/20 rule to quickly get something to market and stem the trickle of Oracle customers who were out of gas on RAC and headed to shared nothing products: Teradata, Netezza, and Greenplum.

This was a very smart move and it worked. Even though the 80/20 approach meant that there were a significant number of queries, the complex queries that needed to process large working sets to execute joins, Exadata solved enough of the problem to keep devout Oracle shops in the church. Only the shops who felt that complex query performance was important enough to warrant the cost of a migration (for an existing DW that had grown up) or the lesser cost of introducing a new technology (for a new DW) would move.

So, while Exadata was a smart move… it is a clear admission that shared-nothing is the right architecture for data warehouses and marts. This admission makes it clear that it is silly to build a warehouse or mart on normal Oracle or on RAC unless you consider your database an inviolable part of a technological creed.

In my opinion selecting a database is an engineering process that does not require orthodoxy… we should be strong enough engineers to pick the better technology and learn it. Being an “Oracle shop” is lazy.

Note that the in-memory technologies provided in Oracle12c are significant… and for warehouses and marts that will fit on a single node, 12c as it matures, will be a fine choice for the orthodox Oracle shop and for others. For bigger data applications you will require Exadata and the limitations that come with it.

This provides a nice transition to the Part 2 post on Teradata and in-memory.

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No Empathy for DevOps

Ugh.

I loved the concept of DevOps and talked it up in the companies I was associated with. Within a database DevOps had a long history as both database products, database ETL facilities, and end-user applications became more dev-operable. The idea that infrastructure had to become code has been a part of the best DBA’s mantra for years.

The cool thing is that you could walk into a first-rate shop and tell that DevOps was part of the infrastructure. You could see it work. If there were two database systems running side-by-side you could determine which system had DevOps components built in.

But somehow the DevOps concept has become a process rather than an outcome. DevOps is no longer an infrastructure as code… it has become a development process, a method, that has qualities like “empathy”. It is “a software development method that stresses communication, collaboration (information sharing and web service usage), integration, automation and measurement between software developers and Information Technology (IT) professionals”. It is a “culture”… that requires corporate management buy-in.

Ugh.

IMO DevOps is a set of software features that provide resiliency. These features are coded specifically for applications… or they are applications architected to be restartable within some larger, software-based infrastructure. Software defined machines (virtual machines), software defined networks, software defined storage sub-systems are all examples of infrastructure that could be coded to provide a self-healing business application. It is these features that we see when we recognize DevOps at work.

It may be true that there is a method that best supports the development of these features… but the evolution of the word “DevOps” from a set of engineered features to a method that focusses on people is not a positive development.

I would suggest that every DBA think about how to add DevOps capabilities to the processes that support your business applications. I suppose that these DBAs should also be empathetic and collaborate with the application developers… but empathy and collaboration are not the measure of a system that is built on the principles of DevOps.

References:

 

Hadoop Squeezes Greenplum

For several years now I have been suggesting that Hadoop will squeeze the big data RDBMSs: Teradata, Exadata, Greenplum, and Netezza… squeezing them first out of the big data end of the market and then impinging on the high-end of the EDW space. Further I have suggested that there may be a significant and immediate TCO reduction from using Hadoop with your EDW RDBMS which squeezes these product’s market faster and further.

Originally I suggested that Greenplum and Netezza would feel the squeeze first since they were embracing Hadoop directly and at the expense of their RDBMS offerings. Greenplum took this further by trying to compete on price… cutting the price of the GPDB and then introducing HAWQ, basically GPDB on HDFS, at a Hadoop DBMS price point. These moves coupled with a neglect of the EDW market where Greenplum made its name apparently has allowed Hadoop to squeeze Greenplum out of the commercial market.

My network has been humming with rumors from reliable sources for 4+ weeks now… and I am now getting confirmation from both inside and outside Pivotal that the Greenplum software will move to open source in short order. The details are being worked out… and while there may still be a change of heart… it seems to be a done deal. The buzzness plan that Greenplum embarked on prior to the EMC acquisition in 2010 has not been a commercial success.

No one is sorrier to see this than me. Greenplum had a real shot at success. It was a very solid piece of work leading the space with strong architectural extensions like data flow shared nothingness, hybrid row/columnar capabilities, and into big data applications. The ORCA optimizer had the potential to change the game again.

Greenplum was nearly profitable in 2009 running hard at Teradata and Exadata and Netezza in the EDW space. The EDW market is tough… so we have to be fair and point out that pursuing this market may have led to the same result… but a small-market analytics play was followed by an open-source Hadoop play that could only end in squeezing Greenplum. There was never really a business plan with a win at the end.

Hopefully by open sourcing Greenplum some of the sound software will make it into PostgreSQL… but dishing Greenplum into the open source space with few developers and no community dishes it into the same space that Informix, Red Brick, and others sit. I know that I suggested open sourcing Greenplum over 18 months ago (see the wacky idea here)… but the idea then, as now, amounts to capitualization. I just declared what seemed to me to be inevitable a little sooner than Pivotal.

Teradata has now further embraced Hadoop… and they run the risk of repeating the Greenplum downturn. They have a much stronger market platform to work from… but in the long run this may also be a deadly embrace.

So here is another wacky idea. The only successful business model around open source software to date (which is not to say that there is not some other model to be discovered) generates revenue from support and services and just a little software around the edges. Teradata has a support team and a services business that knows big data and is embedded in the enterprise… Cloudera, Hortonworks, and MapR are not close here. Were Teradata to go after the Hadoop market with their own distribution (not much of a barrier to entry here.. just download the Apache stuff and build a team of committers… they might even be able to pick up the Pivotal team)… they would start from a spot way ahead of the start-ups in several respects… in several hard respects. Further they have Aster IP which could qualify as software around the edges. As a Hadoop player Teradata could more easily manage how Hadoop squeezes their business, mitigate risk, and emerge a big winner in the big data space.

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Using Teradata’s Appliance for Hadoop to Reduce TCO

Teradata has recently announced a very complete Teradata database-to-Hadoop integration. Is this note we’ll consider how a Teradata shop might effectively use these features to significantly reduce the TCO of any Teradata system.

The Teradata Appliance for Hadoop (here) offering is quite well thought out and complete… including a Teradata appliance, a Hadoop appliance, and the new QueryGrid capability to seamlessly connect the two… so hardware, software, support, and services are all available in very easy-to-consume bundles.
There is little published on the details of the QueryGrid feature… so I cannot evaluate where it stands on the query integration maturity curve (see here)… but it certainly provides a significant advance over the current offering (see here and Dan Graham’s associated comments).
I believe that there is some instant financial gratification to be had by existing Teradata customers from this Hadoop mashup. Let’s consider this…
Before the possibility of a Hadoop annex to Teradata, Teradata customers had no choice but to store cold, old, data in the Teradata database. If, on occasion, you wanted to perform year by year comparisons over ten years of data then you needed to keep ten years of data in the database at a rough cost of $50K/TB (see here) … even if these queries were rarely executed and were not expected to run against a high performance service level requirement. If you wanted to perform some sophisticated predictive analysis against this data it had to be online. If fact, the Teradata mantra… one which I wholeheartedly agree with… suggests that you really should keep the details online forever as the business will almost always find a way to glean value from this history.
This mantra is the basis of what the Hadoop vendors call a data lake. A data warehouse expert would quickly recognize a data lake as a staging area for un-scrubbed detailed data… with the added benefit that a Hadoop-based data lake can store and process data at a $1K/TB price point… and this makes it cost-effective to persist the staged data online forever.
So what does this mean to a Teradata EDW owner? Teradata has published numbers (here) suggesting that 92% of the queries in an EDW only touch 20% of the data. I would suggest that there is some sort of similar ratio that holds for 90% of the remaining queries… they may touch only another 40% of the data. This suggests that the 40% of the data remaining is online to service less than 1% of the queries… and I suggest that these queries can be effectively serviced from the $1K/TB Hadoop annex.
In other words, almost every Teradata shop can immediately benefit from Teradata’s new product announcements by moving 40% of their Teradata database data to Hadoop. Such a move would free Teradata disk space and likely take pressure off to upgrade the cluster. Further, when an upgrade is required, users can reduce the disk footprint of the Teradata database side of the system; add a Hadoop annex, and significantly reduce the TCO of the overall configuration.
Some time back I suggested that Teradata would be squeezed by Hadoop (here and here). To their credit Teradata is going to try and mitigate the squeeze. But the economics remain… and Teradata customers should seriously consider how to leverage the low $/TB of Teradata’s Hadoop offering to reduce costs. Data needs to reside in the lowest cost infrastructure that still provides the required level of service… and the Teradata Hadoop integration provides an opportunity to leverage a new, low-cost, infrastructure.

Logical Data Warehouses and the Basics of Database Federation

This post will consider the implications of a full database federation as would be required by a Logical Data Warehouse. I’ll build on the concepts introduced in the posts on RDBMS-Hadoop integration (Part1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8).

Figure 1 summarizes those earlier concepts from simple to advanced.

2 Tier Federation Maturity
Figure 1. 2 Tier Federation Maturity

But the full federation required to implement a logical data warehouse requires a significant step up from this. Simple federation will be a disaster and Basic federation will not be much better. Here is why.

Let’s add a database and use Figure 2 to consider the possibilities when we submit a query that joins Table A.One to A.Two to B.One to C.One. Note that in this picture we have included a Governor to execute the federated queries that is independent of any of the DBMSs… this is the usual case for federation.

In the simple case where the Governor executes the entire plan all of the data must come to the Governor. This is clearly unacceptable. Consider the worse case where a SELECT is issued against only one table… still all of the data must bubble up.

In the Basic case the problem is partially mitigated… less data moves after the predicates are resolved but the overhead will still kill query performance. A Governor with basic capabilities provides the minimal features to make this work. It is useful where slow federation is better than data replication… but that is about all.

Figure 2. N-Tier Federation
Figure 2. N-Tier Federation

However, the advanced case becomes seriously more complicated. The optimizer now has to decide if table B.One should move to C to join the data or should it move to A… or should it move data to the Governor.

The problem is further complicated by any resource shortage on any node or any functional capability differences. If the cost of data movement would suggest moving B data to C… but there is no CPU resource available on C then maybe a different decision should be made? If C.One is a big table but C is a column-store and the cost of the SELECT is small because a minimum of columns are required and the cardinality of those columns is small so the data might be fetched from the dictionary then we might make a different decision. If B is a fast in-memory database but there is no memory available then the cost changes. Finally, if there are twenty databases in your logical DW then the problem increases exponentially.

The point here is clear… data federation over n-tiers is a hard problem. There will be severe performance issues when the optimizer picks wrong. This is why the independent governor model is so attractive… Many of the variables around CPU resources and database capabilities are removed… and while the performance will be poor it will be predictably poor. You should consider the implications carefully… it is just not clear that a high-performance logical data warehouse is feasible simply laid over an existing architecture. And if you build on a model with a Governor you must be sure that the Governor, and the Provincial databases can handle the load. I suspect that the Governor will have to run on a cluster and use a shared-nothing architecture to handle a true enterprise-sized logical EDW.

HANA has a twist on this that is interesting. The Governor lives inside one of the database nodes… so for data in HANA there is no data movement cost unless the optimizer decides to send the data to another node. Further, HANA is very fast… and the performance will mitigate some of the slowness inherent in federation. Finally, HANA is a shared-nothing DBMS… so it is not a problem to move lots of data to HANA in support of big tables and/or thousands of concurrent queries.

I’ll try to use this thinking: simple, basic, advanced federation over some governed federator on a an in-memory or fast shared-nothing architecture to evaluate the products on the market that provide federation. This may prove interesting as the Logical Data Warehouse concept catches on and as products like Teradata’s QueryGrid come to market.

Part 8 – How Hadooped is SQL Server PDW with Polybase?

Now for SQL Server… continuing the thread on RDBMS-Hadoop integration (Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7) I have suggested that we could evaluate integration architecture using three criteria:

  1. How parallel are the pipes to move data between the RDBMS and the parallel file system;
  2. Is there intelligence to push down predicates; and
  3. Is there more intelligence to push down joins and other relational operators?

Before we start I will suggest a fourth criteria that will be more fully explored later when we consider networks and pipes… that is: how is data sharded/hashed/distributed as it moves from the distribution scheme in HDFS to an optimal, usually hashed, scheme in the target RDBMS. Consider Greenplum as an example… they move data in parallel as quickly as possible to the GPDB and then redistribute the data across GPDB segment nodes using scatter-gather, a very efficient distribution mechanism. We will consider how PDW Poybase manages this as part of our first criteria.

Also note… since I started this series Teradata has come out with a new capability: the QueryGrid. I will add a post to consider this separately… and in this note I will assume the older Teradata capability. This is a little unfair to Teradata and I apologize for that… but otherwise this post becomes too complex. I’ll make things right for Teradata ASAP.

Now on to Microsoft…

First, Polybase has effective parallel pipes to move data from HDFS to the parallel SQL Server instances in PDW. This matches the best capability of other products like Teradata and Greenplum in this category. But where Teradata and Greenplum move data and then redistribute it, pushing the data over a network twice, Poybase has pushed the PDW hash function down to the HDFS node so that data is distributed as it is sent. This very nice feature skips one full move of the data.

Our second criteria considers how smart the connector is in pushing down filters/predicates. Polybase uses a cost-based approach to determine whether is is less expensive to push predicates down or to move all of the data up to the PDW layer. This is a best-in-class capability.

For the 3rd criteria we ask does the architecture push down advanced functions like joins and aggregates… and does the architecture minimize data pulled up to join with semi-joins? Polybase again provides strong capabilities here pushing down joins and aggregates. Polybase does not use semi-joins, so there is room to improve here… but Microsoft clearly has this capability in their roadmap.

One final note… Polybase works with PDW but not with other SQL Server products. This limitation may be relevant in many cases.

PDW + Polybase is a strong offering… matching HANA in most aspects with HANA having a slight edge in push-down with semi-joins but with SQL Server matching this with the most sophisticated parallel data distribution capability.

References

Part 7 – How Hadooped is Greenplum, the Pivotal GPDB?

Now for Greenplum & Hadoop… to continue this thread on RDBMS-Hadoop integration (Part 1, Part 2, Part 3, Part 4, Part 5, Part 6) I have suggested that we could evaluate integration architecture using three criteria:

  1. How parallel are the pipes to move data between the RDBMS and the parallel file system;
  2. Is there intelligence to push down predicates; and
  3. Is there more intelligence to push down joins and other relational operators?

The Greenplum interface is architecturally similar to the Teradata interface described in Part 4. Hadoop files are defined to the DBMS as external tables and there are capable parallel pipes to effectively move data from the HDFS side to GPDB. In addition Greenplum uses their Scatter-Gather method to load data into the GPDB effectively.

There is no ability to push down predicates. When a query executes all of the relevant data is sucked through the parallel pipes into the database segments for processing. This is very inefficient and there is not even the crude capability to push down processing provided by Teradata.

Finally, there is no ability to push down joins or aggregation.

Greenplum’s offering is not very advanced. To perform with Greenplum analytics data must move between the two storage layers with no intelligence to mitigate the cost.

On to the last post in the series Part 8 on SQL Server and Polybase.

Dynamic Late Binding Schemas on Need

I very much like Curt Monash’s posts on dynamic schemas and schema-on-need… here and here are two examples. They make me think… But I am missing something… I mean that sincerely not just as a setup for a critical review. Let’s consider how dynamism is implemented here and there…so that I can ask a question of the audience.

First imagine a simple unschema’d row:

Rob KloppDatabase Fog Bloghttp://robklopp.wordpress.com42

A human with some context could see that there is a name string, a title string, a URL string, and an integer string. If you have the right context you would recognize that the integer holds the answer to the question: “What is the meaning of Life, the Universe, and Everything?”… see here… otherwise you are lost as to the meaning.

If you load this row into a relational database you could leave the schema out and load a string of 57 characters… or load the data parsed into a schema with Name, Title, URL, Answer. If you load this row into a key-value pair you can load it into an unschema’d row with the Key = Row and the Value equal to the string… or parse the data into four key-value pairs.

In any case you have to parse the data… If you store the data in an unschema’d format you have to parse the data and bind value to keys to columns late… if you store the data parsed then this step is unnecessary. To bind the data late in SQL you might create a view from your program… or more likely you would name the values parsed with SQL string functions. To parse the data into key-value pairs you must do the equivalent. The same logic holds true for more complex parsing. A graph database can store keys, values, and relationships… but these facets have to be known and teased out of the data either early or late. An RDBMS can do the same.

So what is the benefit of a database product that proclaims late binding as an advantage? Is it that late binding is easier to do than in an RDBMS? What am I missing?

Please do not respond with a list of other features provided by NewSQL and NoSQL databases… I understand many of the trade-offs… what I want to know is:

  • What can they do connected to binding values to names that an RDBMS cannot? And if there is no new functionality…
  • Is there someway they allow for binding that is significantly easier?

By the way, the Hitchhiker’s Guide is silent on the question of whether 42 is a constant or ever-changing. I think that I’ll ask Watson.

Part 5: A Review of Processing Push-down

Continuing this thread on RDBMS-Hadoop integration (Part 1, Part 2, Part 3, Part 4) I have suggested that we could evaluate integration architecture using three criteria:

  1. How parallel are the pipes to move data between the RDBMS and the parallel file system;
  2. Is there intelligence to push down predicates; and
  3. Is there more intelligence to push down joins and other relational operators?

I want to be sure that I’ve conveyed the concepts behind these criteria properly… I may have rushed it in the early parts of this series.

Let’s imagine a query that joins a 2,000,000 row table with a 1000 row dimension table where both live in HDFS.

If all of the data has to be moved from HDFS to the RDBMS then 2,001,000  rows must be read and moved in order to apply a predicate or any other processing.. For fun lets say that the cost of moving this data is 2001K.

If there are 10 parallel pipes then the data movement is completed in one tenth the time… so the cost is 200K.

If a predicate is included that selects only 5% of the data from the big table, and the predicate is pushed down the cost is reduced to 101K. Add in parallel pipes and the cost is 10K

Imagine a query where there is a join between the two tables with predicates on one side and predicate push down… then you have to pay 101K to pull the projected data up and do the join in the RDBMS. If there is a join predicate that reduces the final answer set by another 95% then after the join you return 6K rows. Since everybody returns the same 6K rows as an answer we won’t add that in.

But if you can push the join down as well as the predicates then only 6K rows are moved up… so you can see how 2001K shrinks to 6K through the effective push down of processing.

Further, you can build arbitrarily complex queries and model them pretty well knowing that most of the cost is in data movement.

So think about how Teradata processes these two tables in Hadoop when you use the specialized SQL constructs and then again if you build the query from a BI tool. And stay tuned as I’ll show you how HANA processes the data next…. and then talk about several others.

On to Part 6…

Part 4: How Hadooped is Teradata?

In this thread on RDBMS-Hadoop integration (Part 1, Part 2, Part 3) I have suggested that we could evaluate integration architecture using three criteria:

  1. How parallel are the pipes to move data between the RDBMS and the parallel file system;
  2. Is there intelligence to push down predicates; and
  3. Is there more intelligence to push down joins and other relational operators?

Let’s consider the Teradata SQL-H implementation using these criteria.

First, Teradata has effective parallel pipes to move data from HDFS to the Teradata database with one pipe per node. There does not seem to be any inter-node IO parallelism. This is a solid feature.

There is a limited ability to push down predicates… SQL-H does allow data to be partitioned on the HDFS side and it will perform partition elimination if the query explicitly calls out a predicate within a partionfilter() keyword. In addition there is an ability to project out columns using a columns() keyword to explicitly specify the columns to be returned. These features are klunky but effective. You would expect partitions to be eliminated when the partitioning column is referenced with a predicate in the query like any other query… and you would expect columns to be projected out if they are not referenced. Normal SQL predicates are applied after the data is moved over the network but before every record is written into the Teradata database.

Finally SQL-H provides no advanced capabilities to push down join operators or other functions.

The bottom line: SQL-H is a sort of klunky implementation, requiring non-ANSI-standard and non-Teradata standard SQL syntax. Predicate push down is limited but better than nothing. As you will see when we review other products, SQL-H is a  basic offering. The lack of full predicate push-down and advanced features will negatively and severely impact performance when accessing large volumes of data, Big Data, and the special SQL syntax will limit the ability to access HDFS data from 3rd party tools. This performance penalty will force customers to pre-join and pre-aggregate data in Hadoop rather than access it naturally.

Next Part 5...

References

Teradata Magazine: Hands On Dynamic Access

Doug Frazier: SQL-H Presentation

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