Databases are as ubiquitous as computers. They are often, erroneously, attributed to data-intensive, long-term storage solutions. Yet in reality they are used in one shape or form in most programs. From word processors to media players to web-browsers. All depend on databases to realize some of their features. This, not mentioning, e-commerce, flight booking, libraries and, of course, government intelligence data. Even when no feature requires a database per-se, user configuration and settings are often stored in databases. Where previously Windows programs depended on the Registry and Unix programs on plain-text files, increasingly new products are utilizing the portable and vastly superior SQLite instead.
Each application of course has its own set of requirements, use-cases and patterns. Yet, surprisingly or otherwise, there are two main patterns that can be said to parent the rest: OLTP and OLAP. Or, Online Transaction Processing and Online Analytical Processing respectively. Consequently, database design and optimization depends heavily on our processing needs and models. (Here I’m not concerned with database engine design and architecture, rather on the use-case patterns and processing models.) To get a better feel of each, let’s consider typical cases for each.
Online Transaction Processing (OLTP)
This type of applications are chiefly characterized by performing real-time, localized, mission-critical operations. That is, the operations concerned must complete virtually immediately (hence real-time processing,) they typically involve a very small number of entries independent of the remaining data (hence localized processing,) and there is zero-tolerance for data inconsistency, corruption or undefined states. The latter property is what requires transactional processing such that either the operation completely succeeds with all its sub-parts or it completely fails restoring the previous state of all involved entries.
In OLTP the operations are mostly that of
Update, with occasional
Delete operations for housekeeping, although typically deletion is differed for performance and/or for later data mining and reporting. The tables involved are typically normalized around the data entities.
Examples include online shopping, flight and medical data and custom data processing.
Online Analytical Processing (OLAP)
In contrast to OLTP, this type of applications are characterized by performing offline, aggregate and low-priority operations. The operations involved here can be executed at low-demand hours, often on archive databases and secondary servers (hence offline processing,) they typically aggregate a very large amount of data to generate statistical data, reports or to find outliers (hence aggregation,) and, since they are offline processing, they are designed to have low-priority, low isolation level (read uncommitted) and, since there is little or no side-effects to failing, they are designed to fail rather than potentially interfere with competing OLTP operations (if executed on the same database.)
OLAP operations are mostly
Select operations with virtually no data modification (except for storing the results, if at all, typically in dedicated tables.) These operations not only aggregate large number of entries, with heavy use of aggregate functions, but they typically join a significant number of tables.
Examples include reporting, billing, statistical analysis and historic data processing.
Similarities and Differences
Since the differences between these two patterns lies in the underlying database operations they perform, it’s no surprise that there aren’t a multitude of basic patterns. The two main operation types are that of read and modify. That is, the applications that mostly read data (OLAP) typically perform massive read operations potentially across the complete data with few or no modification, and those that depend on heavy data modification care most about the integrity of the few entries they modify with little or no interest in other data.
However, one must make note of the fact that modification implies reading. This is crucial for correctly appreciating the performance characteristics of the two patterns. The three data-modifying operations,
Delete, all ought first perform
Select operation(s) in one form or another. While this is more apparent for
Inserts with constraints must first validate that no constraint is violated. This often involves a lookup operation in the constraint index(es). Only in the most basic and simplest cases could an
Insert operation be free of all lookup operations (when foreign key, unique or other constraints are completely absent.)
This doesn’t mean that OLTP operations are a superset of OLAP operations. To try and optimize first and foremost for OLAP with the hope that OLTP operations would naturally also run faster, thanks to the improved read operations that it could utilize, is a fallacy. The two are worlds apart and typically don’t have overlapping indexable data (more on this below.) Where they do share fields, indexes on them would most certainly be welcome to both. Unfortunately, that doesn’t materialize nearly as much as one would like.
|Transactional Processing||Analytical Processing|
|Data||Current and in-progress.||Retired, historic or archived.|
|Typical Operations||Insert, Update, Delete.||Select.|
|Function Types||Based on business requirements.||Based on data mining.|
|Normalization||Highly normalized, entity modeled.||Possibly denormalized to minimize joins.|
|Data Locality||Few tables, few entries.||Large aggregation across many tables.|
|Indexing Strategy||Limited to a few highly-selective fields.||Generous indexing depending on the queries.|
|Operation Duration||Typically very short, sub-millisecond range.||Extended over minutes and hours.|
|Caching Friendliness||Highly volatile data with strict persistence requirements.||Static data, very cache friendly.|
|Backup Requirement||Inconsistency and data loss may cost business.||All operations can be rerun, backup is redundant.|
|Query Complexity||Trivial to simple.||Complex to inhumane and unreadable.|
The above descriptions are certainly not the only possibilities. A combination of both is more reasonable to expect in practice rather than the theoretical and somewhat idealized cases given above. Some such case might include functionality that must aggregate several groups of data without suffering phantom reads. Such a requirement means that not only the aggregate query must run with a high isolation level, reducing parallelism, but that it must also add indexes to finish as fast as possible to free up the entries that concurrent transactions might require. The additional indexes would incur unwelcome cost for data modification (which must also update the indexes as well.)
Due to the differences in the operations involved, and indeed their requirements, optimization in each case is different than the other.
To maximize performance, OLTP operations would use highly selective conditions on normalized tables in queries that are completely indexed (rather than partially indexed). This will get us to the target data as fast as possible, or, in case of insertion, will verify the absence of collisions equally quickly. Typically the primary key is used, which itself may be an alias of the internal row-id.
Next we’d want to localize locks as much as possible to maximize concurrency and minimize collateral impact. Fine grained lock hints such as row-level locks may be used. Normalization helps here by further containing the impact of the transactions as local as possible. In addition, we’d want to minimize the average row-size to improve cache-hits and avoid undue penalty. Once again, Normalization does most of the work here too.
Finally, OLTP operations would want to minimize, if not avoid, indexes, triggers, views and any operation that doesn’t help it reach and update the data as fast as possible. To that end, indexes are only useful to reach the data fast. All others would simply slow down virtually all operations, save for any selects we may have.
For OLAP on the other hand it’d be best to work with denormalized data, to minimize or avoid joins.
Unlike OLTP, it’s very hard to predict or prioritize the conditions most useful to OLAP. This is because depending on the particular query in question, the tables and fields of importance are decided. Indeed, different queries might depend on very different fields and therefore indexes. So indexing some field might be useful to only an isolated query and not others. For a billing system not only the customer and orders are of importance, but the dates of orders and the billing period as well. Similarly, to generate sales reports, the queries involved would select on products and dates. Yet an inventory audit query might depend on a different set of fields. Hence, indexes are best decided based on the concrete queries in question.
From the above it’s quite clear that not only do OLTP and OLAP differ in their operations, but consequently optimizing their queries are apparently in conflict with each other. The requirements for maximizing performance in each case is simply contradictory to one another. But often both types of processing are done in our products. How can we optimize them?
Optimization for both OLTP and OLAP
It must be said from the outset that optimizing for two contradictory set of requirements can be a recipe for disaster. That is, we might end up with worse performance for most queries and even degrade the database design in hope of improving performance. With hubris some might advocate indexing all fields in query conditions, creating an uninformed mess in their wake. In fact, since OLTP operations are inherently very short lived, the overhead of updating superfluous indexes will probably go unnoticed, yet a long-running OLAP operations might get a noticeable boost that the OLAP queries get. From this the gulls in question might pat themselves for a job well done, completely oblivious to the fact that the OLTP operation in question will probably run millions of times, each time incurring the slight cost of the index updates, negating the apparent speed boost. To avoid such scenarios, we must understand that a case of compromise is perhaps unavoidable and approach the problem methodically.
Here are some design patterns that are often employed to maximize performance across both types of queries.
Perhaps the simplest solution is to avoid performing both OLTP and OLAP type operations on the same data-set. To achieve this, we’ll have to split the data over two sets of tables. One, highly normalized and optimized for OLTP and another, perhaps denormalized, OLAP-optimized set.
A data-retirement scheme will be used to move the retired data from the OLTP table(s) into the archive (OLAP) tables. This may be performed using a background process, or using triggers. The latter may not be desirable as triggers would add latency to the OLTP queries, however, on the other hand, the cache hits and the avoidance of extra code and scanning of the OLTP tables might be a good compromise.
A number of application process data in batches. Each batch is processed in transactional fashion and once all the data has been processed completely, reports and other aggregate functions are executed on the final set. Custom data processing is perhaps a good example here, where some data is processed (for example by means of transformation, conversion or other modification) and tracked by transcations in a database. The database accounts for every data item, its states as it is modified with timestamps and warning/error codes where applicable.
Such a processing scheme has the advantage of allowing for an OLTP type optimization for the data processing stage until all the data is completely processed, whereupon OLAP-optimized indexes are dynamically added. The overhead of adding or modifying the indexes (yes, some might be dropped, while new ones added) might win us an overall significant amount of CPU time. Of course we’d need to profile and benchmark thoroughly to decide which indexes to remove and which to add.
Biting the Bullet: Conjoining Both
In some cases we need to aggregate both retired and archived data as well as current and in-transit data. If this scenario is of prime importance, then archiving the data for offline processing might simply be unhelpful, as we must also join the OLTP tables to get as-yet unarchived data. Also, we can’t work in stages and perform dynamic indexing since we have to work with both current and archived data.
For obvious reasons this is the worst of both worlds. In this case, we must very carefully balance two forces:
- Indexes for optimum OLTP performance.
- Indexes for the most important OLAP queries.
Here analytic profiling of the different operations, their execution pattern and respective use-cases is necessary. Once we get a rough idea of the relative importance of our database operations, we can start collecting empirical data using benchmarks to evaluate the different approaches. Since the database design, indexes and other attributes must be shared among all database operations, we must choose the minimum set of indexes etc. that give the maximum performance across all operations.
The difficultly is of course in finding this optimum point. As each index helpful to the OLAP queries, but unnecessary to the OLTP ones, is an overhead for the high-rate transactions. On the other hand, strategic indexes could boost the performance of aggregate queries by orders of magnitude.
Here one is bound to spend a good amount of time using database profiling, statistical analysis and benchmarking. (In a future post I’ll address this topic head-on.) The downside of all this is that once the priorities of our OLAP queries change, then our particular database design will perhaps be outdated too.
The requirements of OLTP and OLAP are as different as are their use-cases. Optimizing for both is a very tricky business, but sometimes unavoidable. If we must maximize performance, we must invest a considerable amount of time designing a well-balanced database schema and, using extensive profiling and benchmarking tools combined with analytic methods, only then can we decide on the design that maximizes performance for our particular needs.
(Anything missing? If you have something interesting on the topic to share, please don’t keep it to yourself.)