Improve count and exists query projections.

We now use COUNT(*) and SELECT 1 for count respective exists queries to enable database optimizers instead of using the key columns.

Closes #1647

Author: mp911de

spring-data-r2dbc/src/main/java/org/springframework/data/r2dbc/core/R2dbcEntityTemplate.java

@@ -247,18 +247,11 @@ public Mono<Long> count(Query query, Class<?> entityClass) throws DataAccessExce
 
 	Mono<Long> doCount(Query query, Class<?> entityClass, SqlIdentifier tableName) {
 
-		RelationalPersistentEntity<?> entity = getRequiredEntity(entityClass);
 		StatementMapper statementMapper = dataAccessStrategy.getStatementMapper().forType(entityClass);
 
 		StatementMapper.SelectSpec selectSpec = statementMapper //
 				.createSelect(tableName) //
-				.doWithTable((table, spec) -> {
-
-					Expression countExpression = entity.hasIdProperty()
-							? table.column(entity.getRequiredIdProperty().getColumnName())
-							: Expressions.just("1");
-					return spec.withProjection(Functions.count(countExpression));
-				});
+				.withProjection(Functions.count(Expressions.just("*")));
 
 		Optional<CriteriaDefinition> criteria = query.getCriteria();
 		if (criteria.isPresent()) {
@@ -284,17 +277,9 @@ public Mono<Boolean> exists(Query query, Class<?> entityClass) throws DataAccess
 
 	Mono<Boolean> doExists(Query query, Class<?> entityClass, SqlIdentifier tableName) {
 
-		RelationalPersistentEntity<?> entity = getRequiredEntity(entityClass);
 		StatementMapper statementMapper = dataAccessStrategy.getStatementMapper().forType(entityClass);
-
-		StatementMapper.SelectSpec selectSpec = statementMapper.createSelect(tableName).limit(1);
-		if (entity.hasIdProperty()) {
-			selectSpec = selectSpec //
-					.withProjection(entity.getRequiredIdProperty().getColumnName());
-
-		} else {
-			selectSpec = selectSpec.withProjection(Expressions.just("1"));
-		}
+		StatementMapper.SelectSpec selectSpec = statementMapper.createSelect(tableName).limit(1)
+				.withProjection(Expressions.just("1"));
 
 		Optional<CriteriaDefinition> criteria = query.getCriteria();
 		if (criteria.isPresent()) {

spring-data-r2dbc/src/test/java/org/springframework/data/r2dbc/core/R2dbcEntityTemplateUnitTests.java

@@ -104,7 +104,7 @@ void shouldCountBy() {
 
         StatementRecorder.RecordedStatement statement = recorder.getCreatedStatement(s -> s.startsWith("SELECT"));
 
-        assertThat(statement.getSql()).isEqualTo("SELECT COUNT(person.id) FROM person WHERE person.THE_NAME = $1");
+				assertThat(statement.getSql()).isEqualTo("SELECT COUNT(*) FROM person WHERE person.THE_NAME = $1");
         assertThat(statement.getBindings()).hasSize(1).containsEntry(0, Parameter.from("Walter"));
     }
 
@@ -145,7 +145,7 @@ void shouldProjectCountResultWithoutId() {
 
         MockResult result = MockResult.builder().row(MockRow.builder().identified(0, Long.class, 1L).build()).build();
 
-        recorder.addStubbing(s -> s.startsWith("SELECT COUNT(1)"), result);
+				recorder.addStubbing(s -> s.startsWith("SELECT COUNT(*)"), result);
 
         entityTemplate.select(WithoutId.class).count() //
                 .as(StepVerifier::create) //
@@ -169,7 +169,7 @@ void shouldExistsByCriteria() {
 
         StatementRecorder.RecordedStatement statement = recorder.getCreatedStatement(s -> s.startsWith("SELECT"));
 
-        assertThat(statement.getSql()).isEqualTo("SELECT person.id FROM person WHERE person.THE_NAME = $1 LIMIT 1");
+				assertThat(statement.getSql()).isEqualTo("SELECT 1 FROM person WHERE person.THE_NAME = $1 LIMIT 1");
         assertThat(statement.getBindings()).hasSize(1).containsEntry(0, Parameter.from("Walter"));
     }
 

spring-data-r2dbc/src/test/java/org/springframework/data/r2dbc/core/ReactiveSelectOperationUnitTests.java

@@ -193,7 +193,7 @@ void shouldSelectExists() {
 
 		StatementRecorder.RecordedStatement statement = recorder.getCreatedStatement(s -> s.startsWith("SELECT"));
 
-		assertThat(statement.getSql()).isEqualTo("SELECT person.id FROM person WHERE person.THE_NAME = $1 LIMIT 1");
+		assertThat(statement.getSql()).isEqualTo("SELECT 1 FROM person WHERE person.THE_NAME = $1 LIMIT 1");
 	}
 
 	@Test // gh-220
@@ -216,7 +216,7 @@ void shouldSelectCount() {
 
 		StatementRecorder.RecordedStatement statement = recorder.getCreatedStatement(s -> s.startsWith("SELECT"));
 
-		assertThat(statement.getSql()).isEqualTo("SELECT COUNT(person.id) FROM person WHERE person.THE_NAME = $1");
+		assertThat(statement.getSql()).isEqualTo("SELECT COUNT(*) FROM person WHERE person.THE_NAME = $1");
 	}
 
 	static class Person {