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polybase-pushdown-computation.md

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description Pushdown computations in PolyBase
title Pushdown computations in PolyBase | Microsoft Docs
dexcription Enable pushdown computation to improve performance of queries on your Hadoop cluster. You can select a subset of rows/columns in an external table for pushdown.
ms.date 11/17/2020
ms.prod sql
ms.technology polybase
ms.topic conceptual
author MikeRayMSFT
ms.author mikeray
ms.reviewer
monikerRange >= sql-server-2016

Pushdown computations in PolyBase

[!INCLUDE sqlserver2016]

Pushdown computation improves the performance of queries on external data sources. Beginning in SQL Server 2016, pushdown computations were available for Hadoop external data sources. SQL Server 2019 introduces pushdown computations for other types of external data sources.

Enable pushdown computation

The following articles include information about configuring pushdown computation for specific types of external data sources:

Select a subset of rows

Use predicate pushdown to improve performance for a query that selects a subset of rows from an external table.

In this example, SQL Server 2016 initiates a map-reduce job to retrieve the rows that match the predicate customer.account_balance < 200000 on Hadoop. Because the query can complete successfully without scanning all of the rows in the table, only the rows that meet the predicate criteria are copied to SQL Server. This saves significant time and requires less temporary storage space when the number of customer balances < 200000 is small in comparison with the number of customers with account balances >= 200000.

SELECT * FROM customer WHERE customer.account_balance < 200000
SELECT * FROM SensorData WHERE Speed > 65;

Select a subset of columns

Use predicate pushdown to improve performance for a query that selects a subset of columns from an external table.

In this query, SQL Server initiates a map-reduce job to pre-process the Hadoop delimited-text file so that only the data for the two columns, customer.name and customer.zip_code, will be copied to SQL Server.

SELECT customer.name, customer.zip_code
FROM customer
WHERE customer.account_balance < 200000

Pushdown for basic expressions and operators

SQL Server allows the following basic expressions and operators for predicate pushdown.

  • Binary comparison operators (<, >, =, !=, <>, >=, <=) for numeric, date, and time values.
  • Arithmetic operators (+, -, *, /, %).
  • Logical operators (AND, OR).
  • Unary operators (NOT, IS NULL, IS NOT NULL).

The operators BETWEEN, NOT, IN, and LIKE might be pushed down. The actual behavior depends on how the query optimizer rewrites the operator expressions as a series of statements that use basic relational operators.

The query in this example has multiple predicates that can be pushed down to Hadoop. SQL Server can push map-reduce jobs to Hadoop to perform the predicate customer.account_balance <= 200000. The expression BETWEEN 92656 AND 92677 is also composed of binary and logical operations that can be pushed to Hadoop. The logical AND in customer.account_balance AND customer.zipcode is a final expression.

Given this combination of predicates, the map-reduce jobs can perform all of the WHERE clause. Only the data that meets the SELECT criteria is copied back to SQL Server.

SELECT * FROM customer 
WHERE customer.account_balance <= 200000 
    AND customer.zipcode BETWEEN 92656 AND 92677

Examples

Force pushdown

SELECT * FROM [dbo].[SensorData]
WHERE Speed > 65
OPTION (FORCE EXTERNALPUSHDOWN);

Disable pushdown

SELECT * FROM [dbo].[SensorData]
WHERE Speed > 65
OPTION (DISABLE EXTERNALPUSHDOWN);

Next steps

For more information about PolyBase, see What is PolyBase?.