Editing Extract, transform, load (section)

=== Performance ===

{{Unreferenced section|date=September 2024}}

ETL vendors benchmark their record-systems at multiple TB (terabytes) per hour (or ~1 GB per second) using powerful servers with multiple CPUs, multiple hard drives, multiple gigabit-network connections, and much memory.

In real life, the slowest part of an ETL process usually occurs in the database load phase. Databases may perform slowly because they have to take care of concurrency, integrity maintenance, and indices. Thus, for better performance, it may make sense to employ:
* ''Direct path extract'' method or bulk unload whenever is possible (instead of querying the database) to reduce the load on source system while getting high-speed extract
* Most of the transformation processing outside of the database
* Bulk load operations whenever possible

Still, even using bulk operations, database access is usually the bottleneck in the ETL process. Some common methods used to increase performance are:
* [[partition (database)|Partition]] tables (and indices): try to keep partitions similar in size (watch for <code>null</code> values that can skew the partitioning)
* Do all validation in the ETL layer before the load: disable [[data integrity|integrity]] checking (<code>disable constraint</code> ...) in the target database tables during the load
* Disable [[database trigger|triggers]] (<code>disable trigger</code> ...) in the target database tables during the load: simulate their effect as a separate step
* Generate IDs in the ETL layer (not in the database)
* Drop the [[database index|indices]] (on a table or partition) before the load – and recreate them after the load (SQL: <code>drop index</code> ...<code>; create index</code> ...)
* Use parallel bulk load when possible – works well when the table is partitioned or there are no indices (Note: attempting to do parallel loads into the same table (partition) usually causes locks – if not on the data rows, then on indices)
* If a requirement exists to do insertions, updates, or deletions, find out which rows should be processed in which way in the ETL layer, and then process these three operations in the database separately; you often can do bulk load for inserts, but updates and deletes commonly go through an [[API]] (using [[SQL]])

Whether to do certain operations in the database or outside may involve a trade-off. For example, removing duplicates using <code>distinct</code> may be slow in the database; thus, it makes sense to do it outside. On the other side, if using <code>distinct</code> significantly (x100) decreases the number of rows to be extracted, then it makes sense to remove duplications as early as possible in the database before unloading data.

A common source of problems in ETL is a big number of dependencies among ETL jobs. For example, job "B" cannot start while job "A" is not finished. One can usually achieve better performance by visualizing all processes on a graph, and trying to reduce the graph making maximum use of [[parallel computing|parallelism]], and making "chains" of consecutive processing as short as possible. Again, partitioning of big tables and their indices can really help.

Another common issue occurs when the data are spread among several databases, and processing is done in those databases sequentially. Sometimes database replication may be involved as a method of copying data between databases – it can significantly slow down the whole process. The common solution is to reduce the processing graph to only three layers:

* Sources
* Central ETL layer
* Targets

This approach allows processing to take maximum advantage of parallelism. For example, if you need to load data into two databases, you can run the loads in parallel (instead of loading into the first – and then replicating into the second).

Sometimes processing must take place sequentially. For example, dimensional (reference) data are needed before one can get and validate the rows for main [[Fact table|"fact" tables]].