Fix v6 migration script for SQL Server

Dropped and re-added unique constraint on BATCH_JOB_INSTANCE
table to optimize performance.

Resolves #5276

Signed-off-by: Beat Jost <beatjost@gmx.ch>

Author: beatjost

spring-batch-core/src/main/resources/org/springframework/batch/core/migration/6.0/migration-sqlserver.sql

@@ -2,10 +2,16 @@
 -- Changes VARCHAR columns to NVARCHAR to align with Microsoft JDBC driver behavior
 -- This improves performance by avoiding implicit conversions and reduces deadlock risk
 
+-- Drop unique constraint on BATCH_JOB_INSTANCE table
+ALTER TABLE BATCH_JOB_INSTANCE DROP CONSTRAINT JOB_INST_UN;
+
 -- BATCH_JOB_INSTANCE table
 ALTER TABLE BATCH_JOB_INSTANCE ALTER COLUMN JOB_NAME NVARCHAR(100) NOT NULL;
 ALTER TABLE BATCH_JOB_INSTANCE ALTER COLUMN JOB_KEY NVARCHAR(32) NOT NULL;
 
+-- ReAdd unique constraint on BATCH_JOB_INSTANCE table
+ALTER TABLE BATCH_JOB_INSTANCE ADD CONSTRAINT JOB_INST_UN UNIQUE (JOB_NAME, JOB_KEY);
+
 -- BATCH_JOB_EXECUTION table
 ALTER TABLE BATCH_JOB_EXECUTION ALTER COLUMN STATUS NVARCHAR(10) NULL;
 ALTER TABLE BATCH_JOB_EXECUTION ALTER COLUMN EXIT_CODE NVARCHAR(2500) NULL;
@@ -31,4 +37,4 @@ ALTER TABLE BATCH_JOB_EXECUTION_CONTEXT ALTER COLUMN SHORT_CONTEXT NVARCHAR(2500
 ALTER TABLE BATCH_JOB_EXECUTION_CONTEXT ALTER COLUMN SERIALIZED_CONTEXT NVARCHAR(MAX) NULL;
 
 -- Rename sequence BATCH_JOB_SEQ to BATCH_JOB_INSTANCE_SEQ
-EXEC sp_rename 'BATCH_JOB_SEQ', 'BATCH_JOB_INSTANCE_SEQ';
+EXEC sp_rename 'BATCH_JOB_SEQ', 'BATCH_JOB_INSTANCE_SEQ';