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## What changes were proposed in this pull request? A logical `Limit` is performed physically by two operations `LocalLimit` and `GlobalLimit`. Most of time, we gather all data into a single partition in order to run `GlobalLimit`. If we use a very big limit number, shuffling data causes performance issue also reduces parallelism. We can avoid shuffling into single partition if we don't care data ordering. This patch implements this idea by doing a map stage during global limit. It collects the info of row numbers at each partition. For each partition, we locally retrieves limited data without any shuffling to finish this global limit. For example, we have three partitions with rows (100, 100, 50) respectively. In global limit of 100 rows, we may take (34, 33, 33) rows for each partition locally. After global limit we still have three partitions. If the data partition has certain ordering, we can't distribute required rows evenly to each partitions because it could change data ordering. But we still can avoid shuffling. ## How was this patch tested? Jenkins tests. Author: Liang-Chi Hsieh <viirya@gmail.com> Closes #16677 from viirya/improve-global-limit-parallelism. |
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| catalyst | ||
| core | ||
| hive | ||
| hive-thriftserver | ||
| create-docs.sh | ||
| gen-sql-markdown.py | ||
| mkdocs.yml | ||
| README.md | ||
Spark SQL
This module provides support for executing relational queries expressed in either SQL or the DataFrame/Dataset API.
Spark SQL is broken up into four subprojects:
- Catalyst (sql/catalyst) - An implementation-agnostic framework for manipulating trees of relational operators and expressions.
- Execution (sql/core) - A query planner / execution engine for translating Catalyst's logical query plans into Spark RDDs. This component also includes a new public interface, SQLContext, that allows users to execute SQL or LINQ statements against existing RDDs and Parquet files.
- Hive Support (sql/hive) - Includes an extension of SQLContext called HiveContext that allows users to write queries using a subset of HiveQL and access data from a Hive Metastore using Hive SerDes. There are also wrappers that allow users to run queries that include Hive UDFs, UDAFs, and UDTFs.
- HiveServer and CLI support (sql/hive-thriftserver) - Includes support for the SQL CLI (bin/spark-sql) and a HiveServer2 (for JDBC/ODBC) compatible server.
Running sql/create-docs.sh generates SQL documentation for built-in functions under sql/site.