Pushdown computations in PolyBase
Applies to: SQL 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.
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:
- Enable pushdown computation in Hadoop
- Configure PolyBase to access external data in Oracle
- Configure PolyBase to access external data in Teradata
- Configure PolyBase to access external data in MongoDB
- Configure PolyBase to access external data with ODBC generic types
- Configure PolyBase to access external data in SQL Server
This table summarizes pushdown computation support on different external data sources:
|Azure Blob Storage||No||No||No||No||Yes|
* Azure Cosmos DB pushdown support is enabled via the Azure Cosmos DB API for MongoDB.
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)|
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 (
- Unary operators (
IS NOT NULL).
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.
Date & time functions
Syntax that prevents pushdown
The following T-SQL functions or syntax will prevent pushdown computation:
AT TIME ZONE
Pushdown support for the
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.
SELECT * FROM [dbo].[SensorData] WHERE Speed > 65 OPTION (FORCE EXTERNALPUSHDOWN);
SELECT * FROM [dbo].[SensorData] WHERE Speed > 65 OPTION (DISABLE EXTERNALPUSHDOWN);
- For more information about PolyBase, see Introducing data virtualization with PolyBase
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