85d77b908b
We optimize plain count(*) queries on Iceberg tables the following way:
AGGREGATE
COUNT(*)
|
UNION ALL
/ \
/ \
/ \
SCAN all ANTI JOIN
datafiles / \
without / \
deletes SCAN SCAN
datafiles deletes
||
rewrite
||
\/
ArithmethicExpr: LHS + RHS
/ \
/ \
/ \
record_count AGGREGATE
of all COUNT(*)
datafiles |
without ANTI JOIN
deletes / \
/ \
SCAN SCAN
datafiles deletes
This optimization consists of two parts:
1 Rewriting count(*) expression to count(*) + "record_count" (of data
files without deletes)
2 In IcebergScanPlanner we only need to consruct the right side of
the original UNION ALL operator, i.e.:
ANTI JOIN
/ \
/ \
SCAN SCAN
datafiles deletes
SelectStmt decides whether we can do the count(*) optimization, and if
so, does the following:
1: SelectStmt sets 'TotalRecordsNumV2' in the analyzer, then during the
expression rewrite phase the CountStarToConstRule rewrites the
count(*) to count(*) + record_count
2: SelectStmt sets "OptimizeCountStarForIcebergV2" in the query context
then IcebergScanPlanner creates plan accordingly.
This mechanism works for simple queries, but can turn on count(*)
optimization in IcebergScanPlanner for all Iceberg V2 tables in complex
queries. Even if only one subquery enables count(*) optimization during
analysis.
With this patch the followings change:
1: We introduce IcebergV2CountStarAccumulator which we use instead of
the ArithmethicExpr. So after rewrite we still know if count(*)
optimization should be enabled for the planner.
2: Instead of using the query context, we pass the information to the
IcebergScanPlanner via the TableRef object.
Testing
* e2e tests
Change-Id: I1940031298eb634aa82c3d32bbbf16bce8eaf874
Reviewed-on: http://gerrit.cloudera.org:8080/23705
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Reviewed-by: Zoltan Borok-Nagy <boroknagyz@cloudera.com>
Welcome to Impala
Lightning-fast, distributed SQL queries for petabytes of data stored in open data and table formats.
Impala is a modern, massively-distributed, massively-parallel, C++ query engine that lets you analyze, transform and combine data from a variety of data sources:
- Best of breed performance and scalability.
- Support for data stored in Apache Iceberg, HDFS, Apache HBase, Apache Kudu, Amazon S3, Azure Data Lake Storage, Apache Hadoop Ozone and more!
- Wide analytic SQL support, including window functions and subqueries.
- On-the-fly code generation using LLVM to generate lightning-fast code tailored specifically to each individual query.
- Support for the most commonly-used Hadoop file formats, including Apache Parquet and Apache ORC.
- Support for industry-standard security protocols, including Kerberos, LDAP and TLS.
- Apache-licensed, 100% open source.
More about Impala
The fastest way to try out Impala is a quickstart Docker container. You can try out running queries and processing data sets in Impala on a single machine without installing dependencies. It can automatically load test data sets into Apache Kudu and Apache Parquet formats and you can start playing around with Apache Impala SQL within minutes.
To learn more about Impala as a user or administrator, or to try Impala, please visit the Impala homepage. Detailed documentation for administrators and users is available at Apache Impala documentation.
If you are interested in contributing to Impala as a developer, or learning more about Impala's internals and architecture, visit the Impala wiki.
Supported Platforms
Impala only supports Linux at the moment. Impala supports x86_64 and has experimental support for arm64 (as of Impala 4.0). Impala Requirements contains more detailed information on the minimum CPU requirements.
Supported OS Distributions
Impala runs on Linux systems only. The supported distros are
- Ubuntu 16.04/18.04
- CentOS/RHEL 7/8
Other systems, e.g. SLES12, may also be supported but are not tested by the community.
Export Control Notice
This distribution uses cryptographic software and may be subject to export controls. Please refer to EXPORT_CONTROL.md for more information.
Build Instructions
See Impala's developer documentation to get started.
Detailed build notes has some detailed information on the project layout and build.