[转帖]SQL Server JDBC – Set sendStringParametersAsUnicode to false

https://vladmihalcea.com/sql-server-jdbc-sendstringparametersasunicode/
https://learn.microsoft.com/en-us/sql/connect/jdbc/setting-the-connection-properties?view=sql-server-ver16

If the sendStringParametersAsUnicode property is set to "true", String parameters are sent to the server in Unicode format.

If the sendStringParametersAsUnicode property is set to "false", String parameters are sent to the server in non-Unicode format such as ASCII/MBCS instead of Unicode.

The default value for the sendStringParametersAsUnicode property is "true".

Note: The sendStringParametersAsUnicode property is only checked to send a parameter value with CHAR, VARCHAR, or LONGVARCHAR JDBC types. The new JDBC 4.0 national character methods, such as the setNString, setNCharacterStream, and setNClob methods of SQLServerPreparedStatement and SQLServerCallableStatement classes, always send their parameter values to the server in Unicode whatever the setting of this property.

For optimal performance with the CHAR, VARCHAR, and LONGVARCHAR JDBC data types, an application should set the sendStringParametersAsUnicode property to "false" and use the setString, setCharacterStream, and setClob non-national character methods of the SQLServerPreparedStatement and SQLServerCallableStatement classes.

When the application sets the sendStringParametersAsUnicode property to "false" and uses a non-national character method to access Unicode data types on the server side (such as nchar, nvarchar and ntext), some data might be lost if the database collation doesn't support the characters in the String parameters passed by the non-national character method.

An application should use the setNString, setNCharacterStream, and setNClob national character methods of the SQLServerPreparedStatement and SQLServerCallableStatement classes for the NCHAR, NVARCHAR, and LONGNVARCHAR JDBC data types.

　　

Last modified: Apr 17, 2021

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Introduction

In this article, I’m going to explain why you should always disable the sendStringParametersAsUnicode default JDBC Driver setting when using SQL Server.

Database table

Let’s assume we have the following database table:

The PostID column is the Primary Key, and the Title column is of the VARCHAR type and has a secondary index as well:

1	CREATE INDEX IDX_Post_Title ON Post (Title)

The Post table contains the following records:

| PostID | Title                                       |

|--------|---------------------------------------------|

| 1      | High-Performance Java Persistence, part 1   |

| 2      | High-Performance Java Persistence, part 2   |

| 3      | High-Performance Java Persistence, part 3   |

| 4      | High-Performance Java Persistence, part 4   |

| ..     | ..                                          |

| 249    | High-Performance Java Persistence, part 249 |

| 250    | High-Performance Java Persistence, part 250 |

As you can see, the Title column is highly selective since every record has a different title value.

Unexpected CONVERT_IMPLICIT and Clustered Index Scan

When finding a Post row by its associated Title column value, we expect an Index Seek operation against the IDX_Post_Title index, but this is not what we get when using the default SQL Server JDBC settings.

For instance, if we enable the runtime query statistics to retrieve the associated execution plan of the SQL query that filters by the Title column:

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executeStatement(entityManager, "SET STATISTICS IO, TIME, PROFILE ON");   try (PreparedStatement statement = connection.prepareStatement("""     SELECT PostId, Title     FROM Post     WHERE Title = ?     """ )) {       statement.setString(1, title);       if (statement.execute() && statement.getMoreResults()) {         LOGGER.info("Execution plan: {}{}",             System.lineSeparator(),             resultSetToString(statement.getResultSet())         );     } }

We get the following SQL execution plan:

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|StmtText                                                                                            | |----------------------------------------------------------------------------------------------------| |SELECT PostId, Title FROM Post WHERE Title = @P0                                                    | |  |--Clustered Index Scan(OBJECT:([high_performance_sql].[dbo].[Post].[PK__Post__AA12603828AEBF55]),| |     WHERE:(CONVERT_IMPLICIT(nvarchar(255),[high_performance_sql].[dbo].[Post].[Title],0)=[@P0]))   |

The Clustered Index Scan operation tells us that SQL Server has used the PostId Clustered Index to scan the leaf pages in search of the Title value we provided.

The reason why the IDX_Post_Title index was not used is because of the implicit conversion that was done between the provided NVARCHAR value and the VARCHAR value of the Title column.

Even if we provided the Title bind parameter value as a VARCHAR using the setString method:

1	statement.setString(1, title);

The SQL Server JDBC Driver behaved as if we used setNString method instead.

SQL Server JDBC sendStringParametersAsUnicode configuration

By default, SQL Server sends all String parameter values as NVARCHAR since the sendStringParametersAsUnicode configuration is set to true.

So, if we set the sendStringParametersAsUnicode configuration value to false

1	jdbc:sqlserver://localhost;instance=SQLEXPRESS;databaseName=high_performance_sql;sendStringParametersAsUnicode=false;

And, rerun the previous SQL query, we will get the following execution plan:

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|StmtText                                                                        | |--------------------------------------------------------------------------------| |SELECT PostId, Title FROM Post WHERE Title = @P0                                | |  |--Index Seek(OBJECT:([high_performance_sql].[dbo].[Post].[IDX_Post_Title]),  | |       SEEK:([high_performance_sql].[dbo].[Post].[Title]=[@P0]) ORDERED FORWARD)|

That’s exactly what we were expecting from the start. There’s an Index Seek on the IDX_Post_Title index, and there’s no implicit conversion happening anymore.

Handing Unicode characters

Now, even if you disable the sendStringParametersAsUnicode setting, you can still persist Unicode data in NHAR, NVARCHAR or NLONGVARCHAR column.

So, if the Title column is of the NVARCHAR type:

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CREATE TABLE Post (     PostID BIGINT NOT NULL,     Title NVARCHAR(255),     PRIMARY KEY (PostID) )

We can set the Title column using the setNString PreparedStatement method:

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try (PreparedStatement statement = connection.prepareStatement("""     INSERT INTO Post (Title, PostID)     VALUES (?, ?)     """ )) {       statement.setNString(1, "România");     statement.setLong(2, 1L);       assertEquals(1, statement.executeUpdate()); }

And, we can read the Title column using the getNString ResultSet method:

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try (PreparedStatement statement = connection.prepareStatement("""     SELECT Title, PostId     FROM Post     WHERE Title = ?     """ )) {       statement.setNString(1, "România");       try(ResultSet resultSet = statement.executeQuery()) {         if (resultSet.next()) {             assertEquals("România", resultSet.getNString(1));             assertEquals(1L, resultSet.getLong(2));         }     } }

If you’re using JPA and Hibernate, the NVARCHAR column needs to be annotated with the @Nationalized Hibernate annotation to instruct Hibernate that the underlying String attribute needs to be handled by the StringNVarcharType, as opposed to the default StringType:

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@Entity(name = "Post") public class Post {       @Id     @Column(name = "PostID")     private Long id;       @Column(name = "Title")     @Nationalized     private String title;       }

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2 Comments on “SQL Server JDBC – Set sendStringParametersAsUnicode to false”

1. Pavel Rund
   March 3, 2023
   

   Thank you for inspiring article, but are you sure about your results? I tried the scenario you described and SQL server have chosen index seek operation even in case of sendStringParametersAsUnicode=true.
   May be it is dependent of SQL server version. I used MSSQL 2017.

   Regards
   Pavel Rund

   Reply
   • vladmihalcea
     March 3, 2023
     

     You’re welcome.

     This test provides the proof.

     Here are the results:

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     Test with sendStringParametersAsUnicode=true   | Rows | Executes | StmtText                                                                                                                                                                                                                    | StmtId | NodeId | Parent | PhysicalOp           | LogicalOp            | Argument                                                                                                                                                                                         | DefinedValues                                                                                                       | EstimateRows | EstimateIO   | EstimateCPU | AvgRowSize | TotalSubtreeCost | OutputList                                                                                                          | Warnings | Type     | Parallel | EstimateExecutions | | ---- | -------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------ | ------ | ------ | -------------------- | -------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------- | ------------ | ------------ | ----------- | ---------- | ---------------- | ------------------------------------------------------------------------------------------------------------------- | -------- | -------- | -------- | ------------------ | | 1    | 1        | SELECT PostId, Title FROM Post WHERE Title = @P0                                                                                                                                                                            | 1      | 1      | 0      |                      |                      |                                                                                                                                                                                                  |                                                                                                                     | 2.0          |              |             |            | 0.0050384817     |                                                                                                                     |          | SELECT   | 0        |                    | | 1    | 1        |   |--Clustered Index Scan(OBJECT:([high_performance_java_persistence].[dbo].[Post].[PK__Post__AA12603836E8D7BA]), WHERE:(CONVERT_IMPLICIT(nvarchar(255),[high_performance_java_persistence].[dbo].[Post].[Title],0)=[@P0])) | 1      | 2      | 1      | Clustered Index Scan | Clustered Index Scan | OBJECT:([high_performance_java_persistence].[dbo].[Post].[PK__Post__AA12603836E8D7BA]), WHERE:(CONVERT_IMPLICIT(nvarchar(255),[high_performance_java_persistence].[dbo].[Post].[Title],0)=[@P0]) | [high_performance_java_persistence].[dbo].[Post].[PostID], [high_performance_java_persistence].[dbo].[Post].[Title] | 2.0          | 0.0046064816 | 4.32E-4     | 61         | 0.0050384817     | [high_performance_java_persistence].[dbo].[Post].[PostID], [high_performance_java_persistence].[dbo].[Post].[Title] |          | PLAN_ROW | 0        | 1.0                |   Test with sendStringParametersAsUnicode=false   | Rows | Executes | StmtText                                                                                                                                                                           | StmtId | NodeId | Parent | PhysicalOp | LogicalOp  | Argument                                                                                                                                                          | DefinedValues                                                                                                       | EstimateRows | EstimateIO | EstimateCPU | AvgRowSize | TotalSubtreeCost | OutputList                                                                                                          | Warnings | Type     | Parallel | EstimateExecutions | | ---- | -------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------ | ------ | ------ | ---------- | ---------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------- | ------------ | ---------- | ----------- | ---------- | ---------------- | ------------------------------------------------------------------------------------------------------------------- | -------- | -------- | -------- | ------------------ | | 1    | 1        | SELECT PostId, Title FROM Post WHERE Title = @P0                                                                                                                                   | 1      | 1      | 0      |            |            |                                                                                                                                                                   |                                                                                                                     | 1.0          |            |             |            | 0.0032831        |                                                                                                                     |          | SELECT   | 0        |                    | | 1    | 1        |   |--Index Seek(OBJECT:([high_performance_java_persistence].[dbo].[Post].[IDX_Post_Title]), SEEK:([high_performance_java_persistence].[dbo].[Post].[Title]=[@P0]) ORDERED FORWARD) | 1      | 2      | 1      | Index Seek | Index Seek | OBJECT:([high_performance_java_persistence].[dbo].[Post].[IDX_Post_Title]), SEEK:([high_performance_java_persistence].[dbo].[Post].[Title]=[@P0]) ORDERED FORWARD | [high_performance_java_persistence].[dbo].[Post].[PostID], [high_performance_java_persistence].[dbo].[Post].[Title] | 1.0          | 0.003125   | 1.581E-4    | 61         | 0.0032831        | [high_performance_java_persistence].[dbo].[Post].[PostID], [high_performance_java_persistence].[dbo].[Post].[Title] |          | PLAN_ROW | 0        | 1.0                |

     So, you can run the test for yourself and see that it works as explained in the article.