Native scoring using the PREDICT T-SQL function
Native scoring uses PREDICT T-SQL function and the native C++ extension capabilities in SQL Server 2017 to generate prediction values or scores for new data inputs in near-real-time. This methodology offers the fastest possible processing speed of forecasting and prediction workloads, but comes with platform and library requirements: only functions from RevoScaleR and revoscalepy have C++ implementations.
Native scoring requires that you have an already trained model. In SQL Server 2017 Windows or Linux, or in Azure SQL Database, you can call the PREDICT function in Transact-SQL to invoke native scoring against new data that you provide as an input parameter. The PREDICT function returns scores over data inputs you provide.
How native scoring works
Native scoring uses native C++ libraries from Microsoft that can read an already trained model, previously stored in a special binary format or saved to disk as raw byte stream, and generate scores for new data inputs that you provide. Because the model is trained, published, and stored, it can be used for scoring without having to call the R or Python interpreter. As such, the overhead of multiple process interactions is reduced, resulting in much faster prediction performance in enterprise production scenarios.
To use native scoring, call the PREDICT T-SQL function and pass the following required inputs:
- A compatible model based on a supported algorithm.
- Input data, typically defined as a SQL query.
The function returns predictions for the input data, together with any columns of source data that you want to pass through.
PREDICT is available on all editions of SQL Server 2017 database engine and enabled by default, including SQL Server Machine Learning Services on Windows, SQL Server 2017 (Windows), SQL Server 2017 (Linux), or Azure SQL Database. You do not need to install R, Python, or enable additional features.
The model must be trained in advance using one of the supported rx algorithms listed below.
If you need to use models from MicrosoftML or microsoftml, use real-time scoring with sp_rxPredict.
Unsupported model types include the following types:
- Models containing other transformations
- Models using the
rxNaiveBayesalgorithms in RevoScaleR or revoscalepy equivalents
- PMML models
- Models created using other open-source or third-party libraries
Example: PREDICT (T-SQL)
In this example, you create a model, and then call the real-time prediction function from T-SQL.
Step 1. Prepare and save the model
Run the following code to create the sample database and required tables.
CREATE DATABASE NativeScoringTest; GO USE NativeScoringTest; GO DROP TABLE IF EXISTS iris_rx_data; GO CREATE TABLE iris_rx_data ( "Sepal.Length" float not null, "Sepal.Width" float not null , "Petal.Length" float not null, "Petal.Width" float not null , "Species" varchar(100) null ); GO
Use the following statement to populate the data table with data from the iris dataset.
INSERT INTO iris_rx_data ("Sepal.Length", "Sepal.Width", "Petal.Length", "Petal.Width" , "Species") EXECUTE sp_execute_external_script @language = N'R' , @script = N'iris_data <- iris;' , @input_data_1 = N'' , @output_data_1_name = N'iris_data'; GO
Now, create a table for storing models.
DROP TABLE IF EXISTS ml_models; GO CREATE TABLE ml_models ( model_name nvarchar(100) not null primary key , model_version nvarchar(100) not null , native_model_object varbinary(max) not null); GO
The following code creates a model based on the iris dataset and saves it to the table named models.
DECLARE @model varbinary(max); EXECUTE sp_execute_external_script @language = N'R' , @script = N' iris.sub <- c(sample(1:50, 25), sample(51:100, 25), sample(101:150, 25)) iris.dtree <- rxDTree(Species ~ Sepal.Length + Sepal.Width + Petal.Length + Petal.Width, data = iris[iris.sub, ]) model <- rxSerializeModel(iris.dtree, realtimeScoringOnly = TRUE) ' , @params = N'@model varbinary(max) OUTPUT' , @model = @model OUTPUT INSERT [dbo].[ml_models]([model_name], [model_version], [native_model_object]) VALUES('iris.dtree','v1', @model) ;
Be sure to use the rxSerializeModel function from RevoScaleR to save the model. The standard R
serialize function cannot generate the required format.
You can run a statement such as the following to view the stored model in binary format:
SELECT *, datalength(native_model_object)/1024. as model_size_kb FROM ml_models;
Step 2. Run PREDICT on the model
The following simple PREDICT statement gets a classification from the decision tree model using the native scoring function. It predicts the iris species based on attributes you provide, petal length and width.
DECLARE @model varbinary(max) = ( SELECT native_model_object FROM ml_models WHERE model_name = 'iris.dtree' AND model_version = 'v1'); SELECT d.*, p.* FROM PREDICT(MODEL = @model, DATA = dbo.iris_rx_data as d) WITH(setosa_Pred float, versicolor_Pred float, virginica_Pred float) as p; go
If you get the error, "Error occurred during execution of the function PREDICT. Model is corrupt or invalid", it usually means that your query didn't return a model. Check whether you typed the model name correctly, or if the models table is empty.
Because the columns and values returned by PREDICT can vary by model type, you must define the schema of the returned data by using a WITH clause.
For a complete solution that includes native scoring, see these samples from the SQL Server development team: