#1404 - Query.findCount() with query.setDistinct(true) ... doesn't give count distinct SQL query

src/main/java/io/ebeaninternal/server/core/DefaultServer.java

@@ -1294,8 +1294,11 @@ public final class DefaultServer implements SpiServer, SpiEbeanServer {
   @Override
   public <T> int findCount(Query<T> query, Transaction t) {
 
-    SpiQuery<T> copy = ((SpiQuery<T>) query).copy();
-    return findCountWithCopy(copy, t);
+    SpiQuery<T> spiQuery = ((SpiQuery<T>) query);
+    if (!spiQuery.isDistinct()) {
+      spiQuery = spiQuery.copy();
+    }
+    return findCountWithCopy(spiQuery, t);
   }
 
   @Override

src/main/java/io/ebeaninternal/server/core/OrmQueryRequest.java

@@ -81,6 +81,8 @@ public final class OrmQueryRequest<T> extends BeanRequest implements SpiOrmQuery
 
   private BeanPropertyAssocMany<?> manyProperty;
 
+  private boolean inlineCountDistinct;
+
   /**
    * Create the InternalQueryRequest.
    */
@@ -742,4 +744,12 @@ public final class OrmQueryRequest<T> extends BeanRequest implements SpiOrmQuery
   public void slowQueryCheck(long executionTimeMicros, int rowCount) {
     ebeanServer.slowQueryCheck(executionTimeMicros, rowCount, query);
   }
+
+  public void setInlineCountDistinct() {
+    inlineCountDistinct = true;
+  }
+
+  public boolean isInlineCountDistinct() {
+    return inlineCountDistinct;
+  }
 }

src/main/java/io/ebeaninternal/server/query/CQueryBuilder.java

@@ -241,10 +241,14 @@ class CQueryBuilder {
 
     ManyWhereJoins manyWhereJoins = query.getManyWhereJoins();
 
-    if (manyWhereJoins.isFormulaWithJoin()) {
-      query.select(manyWhereJoins.getFormulaProperties());
-    } else {
-      query.setSelectId();
+    boolean countDistinct = query.isDistinct();
+    if (!countDistinct) {
+      // minimise select clause for standard count
+      if (manyWhereJoins.isFormulaWithJoin()) {
+        query.select(manyWhereJoins.getFormulaProperties());
+      } else {
+        query.setSelectId();
+      }
     }
 
     CQueryPredicates predicates = new CQueryPredicates(binder, request);
@@ -263,23 +267,35 @@ class CQueryBuilder {
     }
 
     boolean hasMany = sqlTree.hasMany();
-    String sqlSelect = "select count(*)";
-    if (hasMany) {
-      // need to count distinct id's ...
-      query.setSqlDistinct(true);
-      sqlSelect = null;
+
+    String sqlSelect = null;
+    if (countDistinct) {
+      if (sqlTree.isSingleProperty()) {
+        request.setInlineCountDistinct();
+      }
+    } else {
+      if (hasMany) {
+        // need to count distinct id's ...
+        query.setSqlDistinct(true);
+      } else {
+        sqlSelect = "select count(*)";
+      }
     }
 
     SqlLimitResponse s = buildSql(sqlSelect, request, predicates, sqlTree);
     String sql = s.getSql();
-    if (hasMany || query.isRawSql()) {
-      int pos = sql.lastIndexOf(" order by "); // remove order by - mssql does not accept order by in subqueries
-      if (pos != -1) {
-        sql = sql.substring(0, pos);
-      }
-      sql = "select count(*) from ( " + sql + ")";
-      if (selectCountWithAlias) {
-        sql += " as c";
+
+    if (!request.isInlineCountDistinct()) {
+      if (countDistinct) {
+        sql = wrapSelectCount(sql);
+
+      } else if (hasMany || query.isRawSql()) {
+        // remove order by - mssql does not accept order by in subqueries
+        int pos = sql.lastIndexOf(" order by ");
+        if (pos != -1) {
+          sql = sql.substring(0, pos);
+        }
+        sql = wrapSelectCount(sql);
       }
     }
 
@@ -290,6 +306,14 @@ class CQueryBuilder {
     return new CQueryRowCount(queryPlan, request, predicates);
   }
 
+  private String wrapSelectCount(String sql) {
+    sql = "select count(*) from ( " + sql + ")";
+    if (selectCountWithAlias) {
+      sql += " as c";
+    }
+    return sql;
+  }
+
   /**
    * Return the SQL Select statement as a String. Converts logical property
    * names to physical deployment column names.
@@ -536,6 +560,9 @@ class CQueryBuilder {
       if (!useSqlLimiter) {
         sb.append("select ");
         if (query.isDistinctQuery()) {
+          if (request.isInlineCountDistinct()) {
+            sb.append("count(");
+          }
           sb.append("distinct ");
           String distinctOn = select.getDistinctOn();
           if (distinctOn != null) {
@@ -551,6 +578,9 @@ class CQueryBuilder {
       } else {
         sb.append(select.getSelectSql());
       }
+      if (request.isInlineCountDistinct()) {
+        sb.append(")");
+      }
       if (query.isDistinctQuery() && dbOrderBy != null && !query.isSingleAttribute()) {
         // add the orderBy columns to the select clause (due to distinct)
         sb.append(", ").append(DbOrderByTrim.trim(dbOrderBy));

src/main/java/io/ebeaninternal/server/query/SqlTree.java

@@ -159,4 +159,8 @@ class SqlTree {
   boolean hasMany() {
     return manyProperty != null || rootNode.hasMany();
   }
+
+  boolean isSingleProperty() {
+    return rootNode.isSingleProperty();
+  }
 }

src/main/java/io/ebeaninternal/server/query/SqlTreeNode.java

@@ -83,4 +83,8 @@ interface SqlTreeNode {
    */
   ScalarType<?> getSingleAttributeScalarType();
 
+  /**
+   * Return true if the query is known to only have a single property selected.
+   */
+  boolean isSingleProperty();
 }

src/main/java/io/ebeaninternal/server/query/SqlTreeNodeBean.java

@@ -138,6 +138,11 @@ class SqlTreeNodeBean implements SqlTreeNode {
     return false;
   }
 
+  @Override
+  public boolean isSingleProperty() {
+    return properties != null && properties.length == 1 && children.length == 0;
+  }
+
   @Override
   public ScalarType<?> getSingleAttributeScalarType() {
     if (properties == null || properties.length == 0) {

src/main/java/io/ebeaninternal/server/query/SqlTreeNodeExtraJoin.java

@@ -39,6 +39,11 @@ class SqlTreeNodeExtraJoin implements SqlTreeNode {
     this.manyJoin = assocBeanProperty instanceof STreePropertyAssocMany;
   }
 
+  @Override
+  public boolean isSingleProperty() {
+    return false;
+  }
+
   @Override
   public void buildRawSqlSelectChain(List<String> selectChain) {
     // nothing to add

src/main/java/io/ebeaninternal/server/query/SqlTreeNodeManyWhereJoin.java

@@ -36,6 +36,11 @@ class SqlTreeNodeManyWhereJoin implements SqlTreeNode {
     this.parentPrefix = split[0];
   }
 
+  @Override
+  public boolean isSingleProperty() {
+    return false;
+  }
+
   @Override
   public ScalarType<?> getSingleAttributeScalarType() {
     throw new IllegalStateException("No expected");