#3552 Column alias required for SQL Server + findCount + distinct (#3553)

* #3552 Column alias required for SQL Server + findCount + distinct

- Modifies CQueryBuilder with the main fix which is to use the database platform selectCountWithColumnAlias for all distinct queries [and later disable it for single column case]

- Modifies SqlTreeBuilder because we need to NOT use the column alias for the count distinct SINGLE COLUMN case. So change here to:
  - build the root SqlTreeNode first as part of constructor
  - use rootNode.isSingleProperty() to NOT use column alias for the single column case
  - Hence need to initialise DefaultDbSqlContext AFTER the root SqlTreeNode

  - Modifies DefaultDbSqlContext with no effective change, just columnIndex++ ONLY when we are actually using column alias

 - Modifies H2Platform to also use selectCountWithColumnAlias true

* #3552 selectCountWithColumnAlias true for MariaDB and MySql

Plus fix TestRowCount for Oracle platform

ebean-core/src/main/java/io/ebeaninternal/server/query/CQueryBuilder.java

@@ -238,12 +238,14 @@ final class CQueryBuilder {
     query.setMaxRows(0);
 
     boolean countDistinct = query.isDistinct();
+    boolean useColumnAlias = selectCountWithColumnAlias;
     boolean withAgg = false;
     if (!countDistinct) {
       withAgg = includesAggregation(request, query);
       if (!withAgg) {
         // minimise select clause for standard count
         query.setSelectId();
+        useColumnAlias = false;
       }
     }
 
@@ -256,7 +258,7 @@ final class CQueryBuilder {
     }
 
     predicates.prepare(true);
-    SqlTree sqlTree = createSqlTree(request, predicates, selectCountWithColumnAlias && withAgg);
+    SqlTree sqlTree = createSqlTree(request, predicates, useColumnAlias);
     if (SpiQuery.TemporalMode.CURRENT == query.temporalMode()) {
       sqlTree.addSoftDeletePredicate(query);
     }

ebean-core/src/main/java/io/ebeaninternal/server/query/DefaultDbSqlContext.java

@@ -295,9 +295,8 @@ final class DefaultDbSqlContext implements DbSqlContext {
     if (useColumnAlias) {
       sb.append(' ');
       sb.append(columnAliasPrefix);
-      sb.append(columnIndex);
+      sb.append(columnIndex++);
     }
-    columnIndex++;
   }
 
   @Override

ebean-core/src/main/java/io/ebeaninternal/server/query/SqlTreeBuilder.java

@@ -47,7 +47,7 @@ public final class SqlTreeBuilder {
   private final boolean disableLazyLoad;
   private final boolean readOnly;
   private final SpiQuery.TemporalMode temporalMode;
-  private SqlTreeNode rootNode;
+  private final SqlTreeNode rootNode;
   private boolean sqlDistinct;
   private final boolean platformDistinctNoLobs;
   private final SqlTreeCommon common;
@@ -73,6 +73,7 @@ public final class SqlTreeBuilder {
     this.alias = null;
     this.ctx = null;
     this.common = new SqlTreeCommon(temporalMode, disableLazyLoad, readOnly, null);
+    this.rootNode = buildRootNode(desc);
   }
 
   /**
@@ -100,12 +101,13 @@ public final class SqlTreeBuilder {
     this.alias = new SqlTreeAlias(request.baseTableAlias(), temporalMode);
     this.distinctOnPlatform = builder.isPlatformDistinctOn();
     this.platformDistinctNoLobs = builder.isPlatformDistinctNoLobs();
+    this.common = new SqlTreeCommon(temporalMode, disableLazyLoad, readOnly, includeJoin);
+    this.rootNode = buildRootNode(desc);
     String fromForUpdate = builder.fromForUpdate(query);
     CQueryHistorySupport historySupport = builder.historySupport(query);
     CQueryDraftSupport draftSupport = builder.draftSupport(query);
-    String colAlias = subQuery ? null : columnAliasPrefix;
+    String colAlias = subQuery || rootNode.isSingleProperty() ? null : columnAliasPrefix;
     this.ctx = new DefaultDbSqlContext(alias, colAlias, historySupport, draftSupport, fromForUpdate);
-    this.common = new SqlTreeCommon(temporalMode, disableLazyLoad, readOnly, includeJoin);
   }
 
   /**
@@ -113,7 +115,6 @@ public final class SqlTreeBuilder {
    */
   public SqlTree build() {
     // build the appropriate chain of SelectAdapter's
-    buildRoot(desc);
     // build the actual String
     String distinctOn = null;
     String selectSql = null;
@@ -214,8 +215,8 @@ public final class SqlTreeBuilder {
     return ctx.content();
   }
 
-  private void buildRoot(STreeType desc) {
-    rootNode = buildSelectChain(null, null, desc, null);
+  private SqlTreeNode buildRootNode(STreeType desc) {
+    SqlTreeNode root = buildSelectChain(null, null, desc, null);
     if (!rawSql) {
       alias.addJoin(queryDetail.getFetchPaths(), desc);
       alias.addJoin(predicates.predicateIncludes(), desc);
@@ -224,6 +225,7 @@ public final class SqlTreeBuilder {
       alias.buildAlias();
       predicates.parseTableAlias(alias);
     }
+    return root;
   }
 
   /**

ebean-test/src/test/java/org/tests/query/TestRowCount.java

@@ -14,10 +14,10 @@ import java.util.List;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
-public class TestRowCount extends BaseTestCase {
+class TestRowCount extends BaseTestCase {
 
   @Test
-  public void test() {
+  void test() {
     ResetBasicData.reset();
 
     LoggedSql.start();
@@ -46,7 +46,7 @@ public class TestRowCount extends BaseTestCase {
   }
 
   @Test
-  public void find_count_distinct_singleProperty() {
+  void find_count_distinct_singleProperty() {
     ResetBasicData.reset();
 
     Query<Customer> query = DB.find(Customer.class)
@@ -57,13 +57,13 @@ public class TestRowCount extends BaseTestCase {
 
     int count = query.findCount();
 
-    assertThat(sqlOf(query)).contains("select count(distinct t0.anniversary) from o_customer t0 where t0.status = ?");
+    // never see column alias used with count distinct on single column
+    assertThat(query.getGeneratedSql()).contains("select count(distinct t0.anniversary) from o_customer t0 where t0.status = ?");
     assertThat(count).isGreaterThan(0);
   }
 
   @Test
-  public void find_count_distinct_multipleProperties() {
-
+  void find_count_distinct_multipleProperties() {
     ResetBasicData.reset();
 
     Query<Customer> query = DB.find(Customer.class)
@@ -72,7 +72,32 @@ public class TestRowCount extends BaseTestCase {
 
     int count = query.findCount();
 
-    assertThat(sqlOf(query)).contains("select count(*) from ( select distinct t0.anniversary, t0.status from o_customer t0)");
+    if (isSqlServer() || isH2() || isMariaDB() || isMySql()) {
+      assertThat(query.getGeneratedSql()).contains("select count(*) from ( select distinct t0.anniversary c0, t0.status c1 from o_customer t0)");
+    } else {
+      assertThat(query.getGeneratedSql()).contains("select count(*) from ( select distinct t0.anniversary, t0.status from o_customer t0)");
+    }
+    assertThat(count).isGreaterThan(0);
+  }
+
+  @Test
+  void find_count_distinct_multiplePropertiesSameColumn() {
+    ResetBasicData.reset();
+
+    Query<Customer> query = DB.find(Customer.class)
+      .setDistinct(true)
+      .select("id, anniversary, status")
+      .fetch("billingAddress", "id, city")
+      .fetch("shippingAddress", "id, city");
+
+    int count = query.findCount();
+
+    if (isSqlServer() || isH2() || isMariaDB() || isMySql()) {
+      // must use column alias
+      assertThat(query.getGeneratedSql()).contains("select count(*) from ( select distinct t0.anniversary c0, t0.status c1, t1.id c2, t1.city c3, t2.id c4, t2.city c5 from o_customer t0 left join o_address t1 on t1.id = t0.billing_address_id left join o_address t2 on t2.id = t0.shipping_address_id)");
+    } else {
+      assertThat(query.getGeneratedSql()).contains("select count(*) from ( select distinct t0.anniversary, t0.status, t1.id, t1.city, t2.id, t2.city from o_customer t0 left join o_address t1 on t1.id = t0.billing_address_id left join o_address t2 on t2.id = t0.shipping_address_id)");
+    }
     assertThat(count).isGreaterThan(0);
   }
 

platforms/h2/src/main/java/io/ebean/platform/h2/H2Platform.java

@@ -24,6 +24,7 @@ public class H2Platform extends DatabasePlatform {
     this.dbEncrypt = new H2DbEncrypt();
     this.historySupport = new H2HistorySupport();
     this.nativeUuidType = true;
+    this.selectCountWithColumnAlias = true;
     this.supportsDeleteTableAlias = true;
     this.inlineSqlUpdateLimit = true;
     this.dbDefaultValue.setNow("now()");

platforms/mariadb/src/main/java/io/ebean/platform/mariadb/MariaDbPlatform.java

@@ -16,6 +16,7 @@ public class MariaDbPlatform extends BaseMySqlPlatform {
     super();
     this.platform = Platform.MARIADB;
     this.sequenceBatchMode = false;
+    this.selectCountWithColumnAlias = true;
     // for MariaDB probably turn off forwardOnlyHintOnFindIterate with later driver
     // this.forwardOnlyHintOnFindIterate = false;
     this.historySupport = new MariaDbHistorySupport();

platforms/mysql/src/main/java/io/ebean/platform/mysql/BaseMySqlPlatform.java

@@ -13,6 +13,7 @@ public abstract class BaseMySqlPlatform extends DatabasePlatform {
   public BaseMySqlPlatform() {
     super();
     this.useExtraTransactionOnIterateSecondaryQueries = true;
+    this.selectCountWithColumnAlias = true;
     this.selectCountWithAlias = true;
     this.supportsSavepointId = false;
     this.inlineSqlUpdateLimit = true;