Support SimpleExpression as order by expression

To allow sorting on arbitrary SQL, for example the column index instead
of a name. But also more advanced things like functions that can't be
expressed using a `SimpleFunction`.

    orderBy(OrderByField.from(Expressions.just("1")).asc()) => ORDER BY 1 ASC

Author: koenpunt

spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/render/OrderByClauseVisitor.java

@@ -16,6 +16,7 @@
 package org.springframework.data.relational.core.sql.render;
 
 import org.springframework.data.relational.core.sql.Column;
+import org.springframework.data.relational.core.sql.Expressions;
 import org.springframework.data.relational.core.sql.OrderByField;
 import org.springframework.data.relational.core.sql.SimpleFunction;
 import org.springframework.data.relational.core.sql.Visitable;
@@ -81,6 +82,11 @@ Delegation enterNested(Visitable segment) {
 			return Delegation.delegateTo((SimpleFunctionVisitor)delegate);
 		}
 
+		if (segment instanceof Expressions.SimpleExpression) {
+			delegate = new ExpressionVisitor(context);
+			return Delegation.delegateTo((ExpressionVisitor)delegate);
+		}
+
 		return super.enterNested(segment);
 	}
 
@@ -90,6 +96,10 @@ Delegation leaveNested(Visitable segment) {
 			builder.append(delegate.getRenderedPart());
 		}
 
+		if (delegate instanceof ExpressionVisitor) {
+			builder.append(delegate.getRenderedPart());
+		}
+
 		if (segment instanceof Column) {
 			builder.append(NameRenderer.fullyQualifiedReference(context, (Column) segment));
 		}

spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/OrderByClauseVisitorUnitTests.java

@@ -19,6 +19,8 @@
 
 import org.junit.jupiter.api.Test;
 import org.springframework.data.relational.core.sql.Column;
+import org.springframework.data.relational.core.sql.Expression;
+import org.springframework.data.relational.core.sql.Expressions;
 import org.springframework.data.relational.core.sql.OrderByField;
 import org.springframework.data.relational.core.sql.SQL;
 import org.springframework.data.relational.core.sql.Select;
@@ -111,4 +113,20 @@ void shouldRenderOrderBySimpleFunction() {
 		assertThat(visitor.getRenderedPart().toString()).isEqualTo("GREATEST(emp.id, emp.name) ASC");
 	}
 
+	@Test // GH-1348
+	void shouldRenderOrderBySimpleExpression() {
+
+		Table employee = SQL.table("employee").as("emp");
+		Column column = employee.column("name");
+
+		Expression simpleExpression = Expressions.just("1");
+
+		Select select = Select.builder().select(column).from(employee).orderBy(OrderByField.from(simpleExpression).asc())
+				.build();
+
+		OrderByClauseVisitor visitor = new OrderByClauseVisitor(new SimpleRenderContext(NamingStrategies.asIs()));
+		select.visit(visitor);
+
+		assertThat(visitor.getRenderedPart().toString()).isEqualTo("1 ASC");
+	}
 }