Pushdown computations in PolyBase

Applies to: yesSQL Server 2016 (13.x) and later

Pushdown computation improves the performance of queries on external data sources. Beginning in SQL Server 2016 (13.x), pushdown computations were available for Hadoop external data sources. SQL Server 2019 (15.x) introduced pushdown computations for other types of external data sources.

Note

To determine whether or not PolyBase pushdown computation is benefiting your query, see How to tell if external pushdown occurred.

Enable pushdown computation

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

This table summarizes pushdown computation support on different external data sources:

Data Source Joins Projections Aggregations Filters Statistics
Generic ODBC Yes Yes Yes Yes Yes
Oracle Yes Yes Yes Yes Yes
SQL Server Yes Yes Yes Yes Yes
Teradata Yes Yes Yes Yes Yes
MongoDB* No Yes No No Yes
Hadoop No Yes Some** Some** Yes
Azure Blob Storage No No No No Yes

* Azure Cosmos DB pushdown support is enabled via the Azure Cosmos DB API for MongoDB.

** See Pushdown computation and Hadoop providers.

Note

Pushdown computation can be blocked by some T-SQL syntax. For more information, review Syntax that prevents pushdown.

Pushdown computation and Hadoop providers

PolyBase currently supports two Hadoop providers: Hortonworks Data Platform (HDP) and Cloudera Distributed Hadoop (CDH). There are no differences between the two features in terms of pushdown computation.

To use the computation pushdown functionality with Hadoop, the target Hadoop cluster must have the core components of HDFS, YARN and MapReduce, with the job history server enabled. PolyBase submits the pushdown query via MapReduce and pulls status from the job history server. Without either component, the query fails.

Some aggregation must occur after the data reaches SQL Server. But a portion of the aggregation occurs in Hadoop. This method is common in computing aggregations in massively parallel processing systems.

Hadoop providers support the following aggregations and filters.

Aggregations Filters (binary comparison)
Count_Big NotEqual
Sum LessThan
Avg LessOrEqual
Max GreaterOrEqual
Min GreaterThan
Approx_Count_Distinct Is
IsNot

Key beneficial scenarios of pushdown computation

With PolyBase pushdown computation, you can delegate computation tasks to external data sources. This reduces the workload on the SQL Server instance and can significantly improve performance.

SQL Server can push joins, projections, aggregations, and filters to external data sources to take advantage of remote compute and restrict the data sent over the network.

Pushdown of joins

In many cases, PolyBase can facilitate pushdown of the join operator which will greatly improve performance.

If the join can be done at the external data source, this reduces the amount of data movement and improves the query's performance. Without join pushdown, the data from the tables to be joined must be brought locally into tempdb, then joined.

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 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;

Supported functions for pushdown

SQL Server allows the following functions for predicate pushdown.

String functions

  • CONCAT
  • DATALENGTH
  • LEN
  • LIKE
  • LOWER
  • LTRIM
  • RTRIM
  • SUBSTRING
  • UPPER

Mathematical functions

  • ABS
  • ACOS
  • ASIN
  • ATAN
  • CEILING
  • COS
  • EXP
  • FLOOR
  • POWER
  • SIGN
  • SIN
  • SQRT
  • TAN

General functions

  • COALESCE
  • NULLIF

Date & time functions

  • DATEADD
  • DATEDIFF
  • DATEPART

Syntax that prevents pushdown

The following T-SQL functions or syntax will prevent pushdown computation:

  • AT TIME ZONE
  • CONCAT_WS
  • TRANSLATE
  • RAND
  • CHECKSUM
  • BINARY_CHECKSUM
  • ISJSON
  • JSON_VALUE
  • JSON_QUERY
  • JSON_MODIFY
  • NEWID
  • STRING_ESCAPE
  • COMPRESS
  • DECOMPRESS
  • GREATEST
  • LEAST
  • PARSE

Pushdown support for the FORMAT and TRIM syntax was introduced in SQL Server 2019 (15.x) CU10.

Filter clause with variable

If you are specifying a variable in a filter clause, by default this will prevent pushdown of the filter clause. For example, if you run the following query, the filter clause will not be pushed down:

DECLARE @BusinessEntityID INT

SELECT * FROM [Person].[BusinessEntity]  
WHERE BusinessEntityID = @BusinessEntityID;

To achieve pushdown of the variable, you need to enable query optimizer hotfixes functionality. This can be done in any of the following ways:

  • Instance Level: Enable trace flag 4199 as a startup parameter for the instance
  • Database Level: In the context of the database that has the PolyBase external objects, execute ALTER DATABASE SCOPED CONFIGURATION SET QUERY_OPTIMIZER_HOTFIXES = ON
  • Query level: Use query hint OPTION (QUERYTRACEON 4199) or OPTION (USE HINT ('ENABLE_QUERY_OPTIMIZER_HOTFIXES'))

This limitation applies to execution of sp_executesql.

Note: The ability to pushdown the variable was first introduced in SQL Server 2019 CU5.

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

See also