make SelectGenerator capable of using 'insert ... returning ...'

hibernate-core/src/main/java/org/hibernate/dialect/Dialect.java

@@ -3394,10 +3394,10 @@ public abstract class Dialect implements ConversionContext {
 	}
 
 	/**
-	 * Certain dialects support a subset of {@code ScrollModes}.
+	 * Certain dialects support a subset of {@link ScrollMode}s.
 	 * Provide a default to be used by Criteria and Query.
 	 *
-	 * @return ScrollMode
+	 * @return the default {@link ScrollMode} to use.
 	 */
 	public ScrollMode defaultScrollMode() {
 		return ScrollMode.SCROLL_INSENSITIVE;
@@ -3408,7 +3408,7 @@ public abstract class Dialect implements ConversionContext {
 	 * For example:
 	 * {@code select * from Table1 where col1 in (select col1 from Table2 order by col2 limit 1 offset 1)}
 	 *
-	 * @return boolean
+	 * @return {@code true} if it does
 	 */
 	public boolean supportsOffsetInSubquery() {
 		return false;
@@ -3419,7 +3419,7 @@ public abstract class Dialect implements ConversionContext {
 	 * For example:
 	 * {@code select * from Table1 where col1 in (select col1 from Table2 order by col2 limit 1)}
 	 *
-	 * @return boolean
+	 * @return {@code true} if it does
 	 */
 	public boolean supportsOrderByInSubquery() {
 		return true;
@@ -3430,12 +3430,24 @@ public abstract class Dialect implements ConversionContext {
 	 * For example:
 	 * {@code select col1, (select col2 from Table2 where ...) from Table1}
 	 *
-	 * @return boolean
+	 * @return {@code true} if it does
 	 */
 	public boolean supportsSubqueryInSelect() {
 		return true;
 	}
 
+	/**
+	 * Does this dialect fully support {@code insert ... returning ...} in some form?
+	 *
+	 * @return {@code true} if {@link org.hibernate.id.insert.InsertReturningDelegate}
+	 *         works for any sort of primary key column (not just identity columns), or
+	 *         {@code false} if {@code InsertReturningDelegate} does not work, or only
+	 *         works for specialized identity/"autoincrement" columns
+	 */
+	public boolean supportsInsertReturning() {
+		return false;
+	}
+
 	/**
 	 * Does this dialect support the given fetch clause type.
 	 *

hibernate-core/src/main/java/org/hibernate/dialect/MariaDBDialect.java

@@ -22,7 +22,6 @@ import org.hibernate.query.spi.QueryEngine;
 import org.hibernate.service.ServiceRegistry;
 import org.hibernate.sql.ast.SqlAstTranslator;
 import org.hibernate.sql.ast.SqlAstTranslatorFactory;
-import org.hibernate.sql.ast.spi.SqlAppender;
 import org.hibernate.sql.ast.spi.StandardSqlAstTranslatorFactory;
 import org.hibernate.sql.ast.tree.Statement;
 import org.hibernate.sql.exec.spi.JdbcOperation;
@@ -236,6 +235,11 @@ public class MariaDBDialect extends MySQLDialect {
 		return false;
 	}
 
+	@Override
+	public boolean supportsInsertReturning() {
+		return true;
+	}
+
 	@Override
 	public IdentifierHelper buildIdentifierHelper(IdentifierHelperBuilder builder, DatabaseMetaData dbMetaData)
 			throws SQLException {

hibernate-core/src/main/java/org/hibernate/dialect/PostgreSQLDialect.java

@@ -1218,6 +1218,11 @@ public class PostgreSQLDialect extends Dialect {
 		return true;
 	}
 
+	@Override
+	public boolean supportsInsertReturning() {
+		return true;
+	}
+
 	@Override
 	public boolean supportsOffsetInSubquery() {
 		return true;

hibernate-core/src/main/java/org/hibernate/id/SelectGenerator.java

@@ -12,6 +12,7 @@ import org.hibernate.dialect.Dialect;
 import org.hibernate.generator.InDatabaseGenerator;
 import org.hibernate.id.factory.spi.StandardGenerator;
 import org.hibernate.id.insert.InsertGeneratedIdentifierDelegate;
+import org.hibernate.id.insert.InsertReturningDelegate;
 import org.hibernate.id.insert.UniqueKeySelectingDelegate;
 import org.hibernate.persister.entity.EntityPersister;
 import org.hibernate.service.ServiceRegistry;
@@ -100,7 +101,15 @@ public class SelectGenerator
 	@Override
 	public InsertGeneratedIdentifierDelegate getGeneratedIdentifierDelegate(PostInsertIdentityPersister persister) {
 		Dialect dialect = persister.getFactory().getJdbcServices().getDialect();
-		return new UniqueKeySelectingDelegate( persister, dialect, getUniqueKeyPropertyName( persister ) );
+		if ( dialect.supportsInsertReturning() ) {
+			//TODO: this is not quite right, since TableInsertReturningBuilder and then TableInsertStandard
+			//      ultimately end up calling the SqlAstTranslator to generate the SQL which on H2 delegates
+			//      back to IdentityColumnSupport, and this just might not be an identity column
+			return new InsertReturningDelegate( persister, dialect );
+		}
+		else {
+			return new UniqueKeySelectingDelegate( persister, dialect, getUniqueKeyPropertyName( persister ) );
+		}
 	}
 
 	@Override