Amazon Redshift is a fast, fully managed, petabyte-scale data warehousing service that makes it simple and cost-effective to analyze all of your data. Many of our customers—including Boingo Wireless, Scholastic, Finra, Pinterest, and Foursquare—migrated to Amazon Redshift and achieved agility and faster time to insight, while dramatically reducing costs.
Query optimization and the need for accurate estimates
When a SQL query is submitted to Amazon Redshift, the query optimizer is in charge of generating all the possible ways to execute that query, and picking the fastest one. This can mean evaluating the cost of thousands, if not millions, of different execution plans.
The plan cost is calculated based on estimates of the data characteristics. For example, the characteristics could include the number of rows in each base table, the average width of a variable-length column, the number of distinct values in a column, and the most common values in a column. These estimates (or “statistics”) are computed in advance by running an ANALYZE command, and stored in the system catalog.
How do the query optimizer and ANALYZE work together?
An ideal scenario is to run ANALYZE after every ETL/ingestion job. This way, when running your workload, the query optimizer can use up-to-date data statistics, and choose the most optimal execution plan, given the updates.
However, running the ANALYZE command can add significant overhead to the data ingestion scripts. This can lead to customers not running ANALYZE on their data, and using default or stale estimates. The end result is usually the optimizer choosing a suboptimal execution plan that runs for longer than needed.
When you run a SQL query, the query optimizer requests statistics only on columns used in predicates in the SQL query (join predicates, filters in the WHERE clause and GROUP BY clauses). Consider the following query:
In the query above, the optimizer requests statistics only on columns ‘state’ and ‘deptname’, but not on ‘salary’ and ‘hiredate’. If present, statistics on columns ‘salary’ and ‘hiredate’ are ignored, as they do not impact the cost of the execution plans considered.
Based on the optimizer functionality described earlier, the Amazon Redshift ANALYZE command has been updated to optionally collect information only about columns used in previous queries as part of a filter, join condition or a GROUP BY clause, and columns that are part of distribution or sort keys (predicate columns). There’s a recently introduced option for the ANALYZE command that only analyzes predicate columns:
By having Amazon Redshift collect information about predicate columns automatically, and analyzing those columns only, you’re able to reduce the time to run ANALYZE. For example, during the execution of the 99 queries in the TPC-DS workload, only 203 out of the 424 total columns are predicate columns (approximately 48%). By analyzing only the predicate columns for such a workload, the execution time for running ANALYZE can be significantly reduced.
From my experience in the data warehousing space, I have observed that about 20% of columns in a typical use case are marked predicate. In such a case, running ANALYZE PREDICATE COLUMNS can lead to a speedup of up to 5x relative to a full ANALYZE run.
If no information on predicate columns exists in the system (for example, a new table that has not been queried yet), ANALYZE PREDICATE COLUMNS collects statistics on all the columns. When queries on the table are run, Amazon Redshift collects information about predicate column usage, and subsequent runs of ANALYZE PREDICATE COLUMNS only operates on the predicate columns.
If the workload is relatively stable, and the set of predicate columns does not expand continuously over time, I recommend replacing all occurrences of the ANALYZE command with ANALYZE PREDICATE COLUMNS commands in your application and data ingestion code.
Using the Analyze/Vacuum utility
Several AWS customers are using the Analyze/Vacuum utility from the Redshift-Utils package to manage and automate their maintenance operations. By passing the –predicate-cols option to the Analyze/Vacuum utility, you can enable it to use the ANALYZE PREDICATE COLUMNS feature, providing you with the significant changes in overhead in a completely seamless manner.
Enhancements to logging for ANALYZE operations
When running ANALYZE with the PREDICATE COLUMNS option, the type of analyze run (Full vs Predicate Column), as well as information about the predicate columns encountered, is logged in the stl_analyze view:
AWS also enhanced the pg_statistic catalog table with two new pieces of information: the time stamp at which a column was marked as “predicate”, and the time stamp at which the column was last analyzed.
The Amazon Redshift documentation provides a view that allows a user to easily see which columns are marked as predicate, when they were marked as predicate, and when a column was last analyzed. For example, for the emp table used above, the output of the view could be as follows:
After loading new data into an Amazon Redshift cluster, statistics need to be re-computed to guarantee performant query plans. By learning which column statistics are actually being used by the customer’s workload and collecting statistics only on those columns, Amazon Redshift is able to significantly reduce the amount of time needed for table maintenance during data loading workflows.
Be sure to check out the Top 10 Tuning Techniques for Amazon Redshift, and the Advanced Table Design Playbook: Distribution Styles and Distribution Keys.
About the Author
George Caragea is a Senior Software Engineer with Amazon Redshift. He has been working on MPP Databases for over 6 years and is mainly interested in designing systems at scale. In his spare time, he enjoys being outdoors and on the water in the beautiful Bay Area and finishing the day exploring the rich local restaurant scene.
Bigdata and data center