While working on various data warehousing or batch jobs I have pondered, ran proof of concept projects for evaluating job scheduling tools, and implemented jobs so that they could be run in minimal time utilizing all the resources available in the execution environment while avoiding pitfalls like excessive parallelism. I have come up with some observations/ideals/requirement in this area, as described below.
Continued from Columnar Databases I ...
So if a situation allows us to live with the limitations of the columnar databases, how good are columnar databases. To find out for myself I set up an experiment to compare a popular row based database (Oracle 11g) with compression turned on with a columnar database (Infobright) that relies on an open source database engine (MySQL). I also set the experiment to mainly explore the compression in storage, rather than any query performance as I did not have resource to set up that elaborate an experiment.
For OLTP data structures(TCP-H(tm) Bench Mark) the Oracle compressed row data store used about 10 GB storage including some indexes, which are required for such databases. Infobright database on the other hand showed a 1.9GB. This is about 1/5th the storage required. This is a significant saving, when there are not a lot of indexes in row store, and if more indexes were added for performance reasons it would have shown even better comparison on storage requirements.
For Star Schema Bench Mark database, the data extracts were of the range of 6.7 GB of raw ASCII data, when pulled from Infobright (it by default provides quoted strings, etc) vs. about 6 GB of raw ASCII data when pulled from Oracle tables, using custom pipe delimiter. When loaded into Infobright it compressed the data into a size of about 800MB, again using no indexes. When loaded into Oracle database with the same compression scheme as before the data used about 6.5GB. From these observations, we conclude that while Oracle provides compression, considering that we had 3 large indexes on the lineorder, and smaller indexes on smaller tables as well. However the columnar database (Inforbright) provided an order of magnitude compression compared to the raw text data and row store's (Oracle) equivalent database with basic compression. Due to lack of appropriate storage (exadata machine) I could not test the more aggressive compression scheme available from Oracle row store database.
Query timings were better in case of Infobright database where the large fact table extraction to flat file took about 12 minutes and in case of Oracle the same took about 42 minutes. Thus highlighting the benefits of smaller storage, at the least, as the queries did not use any index for Oracle either as these queries gets all the data from the tables in the join.
The star schema shows a higher compression ratio for the columnar database, even though, it uses mainly numeric type data types in the large fact table.
Abadi, D., Boncz, P., Harizopoulos, S. "Column-oriented Database Systems" in VLDB ’09, August 24-28, 009, Lyon, France.
Abadi, D.J., Madden, S.R., and Ferreira, M. "Integrating compression and execution in column-oriented database systems" In Proc. SIGMOD, 2006.
Hodak, W., Jernigan, Kevin, "Advanced Compression with Oracle Database 11g Release 2" An Oracle White Paper from Oracle corporation, September 2009
Oracle, "Oracle 11g SQL Reference Guide" from otn.oracle.com
Oracle, "Oracle 11g Utilities Guide" from otn.oracle.com
Inforbright.org, "Direct data load guide" available from inforbright.org
With the need for processing more and more data and also the availability for more data captured electronically from various data collection points through commercial, non-profit, government or research communities. This phenomenon is termed as Big Data in industry parlance. To make sense out of
this data being gathered it requires large amount of processing power. This data may be available in granular form or as documents, and sometimes both may be co-related. Over the period of time we notice that the nature of the data gathered is getting changed. Traditionally most of the data was transactional in nature, requiring CRUD(create, update, delete) operations. Now a larger amount of data is being created that is usually not updated and may only be deleted when it is no longer needed, usually after a longer period of time than in the transactional sense. While OLTP database provided the ability to store the CRUD operations with ACID(atomic, consistent, isolated and durable) properties for handling more granular data, they were then enhanced to add storage of various types of documents(text, pictures, etc.) again with the OLTP type of transactions in mind. These databases typically use a normalized data model for storage. But the need for providing ACID guaranties, and to handle different type of data volumes for analytical needs, the data could no longer be contained in those models. Therefore the data warehouses were designed using same type of databases, but with different type of data models (typically dimensional, though not always). Data warehouses allowed separation of data from the OLTP systems, but still grew fairly large in volumes, and typically serve more read type operations than frequent updates or writes.
While working with large volumes of data, I noticed that at times a large number of columns in a table have low cardinality, but the overall size of the table itself may be fairly large. This led me to believe that one could reduce the size of the data as stored on the disk if compression techniques are used. Since disk access is usually the slowest part of the access of a database a smaller footprint of data would presumably lead to faster retrieval of the data from the slower medium, however, there would be associated CPU cost that would be incurred in compressing the data. Since data warehouses carry the largest amounts of data there presumably would be tradeoff scenarios in using one or the other technique. Even though the normalization theory is about reducing the redundancy of duplicate data, and therefore providing most efficient storage, there has to be other techniques that could be combine with this to reduce the overall query timing. One of the recent technologies that focus on this aspect is the columnar storage based databases.
[Abadi, Boncz, Harizopoulos, 2009] provide a brief introductory, tutorial to the columnar databases. They describe the columnar databases as
"Column-stores, in a nutshell, store each database table column separately, with attribute values belonging to the same column stored contiguously, compressed, and densely packed, as opposed to traditional database systems that store entire records (rows) one after the other."
They trace the history of the column stores back to 1970s, when the usage of transposed files were explored. In 1980s the benefits of decomposed storage mode(DSM) over row based storage were explored. Its only in 2000s that these data stores finally took off.
Because of the affinity of the data values stored contiguously on disk pages for each of the columns the data lends to better compression schemes, that may be light weight in their CPU utilization but still provide heavy
However, these databases are challenged in their ability to provide updates and also in tuple construction required for use in applications which access data through ODBC/JDBC type interfaces. The tuple construction is required to present the data in row format used by these access applications.
Continued at Columnar Databases II
About Sarbjit Parmar
A practitioner with technical and business knowledge in areas of Data Management( Online transaction processing, data modeling(relational, hierarchical, dimensional, etc.), S/M/L/XL/XXL & XML data, application design, batch processing, analytics(reporting + some statistical analysis), MBA+DBA), Project Management / Product/Software Development Life Cycle Management.