[GitHub] [shardingsphere] sandynz commented on pull request #21992: Improve AbstractMemoryQueryResult.wasNull

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sandynz commented on PR #21992:
URL: https://github.com/apache/shardingsphere/pull/21992#issuecomment-1325861721

   > When call `next` method,
   > 
   > ```java
   > public final boolean next() {
   >     if (rows.hasNext()) {
   >         currentRow = rows.next();
   >         rowCount--;
   >         return true;
   >     }
   >    // when rows.hasNext() is false, this line will set currentRow = null and `wasNull()` method will return true before,but return false now.
   >     currentRow = null;
   >     return false;
   > }
   > ```
   > 
   > when rows.hasNext() is false, this line will set currentRow = null and `wasNull()` method will return true before,but return false now.
   > 
   > wasNull method before:
   > 
   > ```java
   >   public final boolean wasNull() {
   >      // return true;
   >       return null == currentRow;
   >   }
   > ```
   > 
   > wasNull method now:
   > 
   > ```java
   >   public final boolean wasNull() {
   >           // return false;
   >       return wasNull;
   >   }
   > ```
   
   Hi @FlyingZC , some investigation as following.
   
   The definition in JDBC specification of `wasNull`:
   ```
       /**
        * Reports whether
        * the last column read had a value of SQL <code>NULL</code>.
        * Note that you must first call one of the getter methods
        * on a column to try to read its value and then call
        * the method <code>wasNull</code> to see if the value read was
        * SQL <code>NULL</code>.
        *
        * @return <code>true</code> if the last column value read was SQL
        *         <code>NULL</code> and <code>false</code> otherwise
        * @exception SQLException if a database access error occurs or this method is
        *            called on a closed result set
        */
       boolean wasNull() throws SQLException;
   ```
   
   MySQL JDBC implementation of `wasNull` (mysql-connector-java:5.1.47):
   ResultSetImpl.java
   ```
       /** Did the previous value retrieval find a NULL? */
       protected boolean wasNullFlag = false;
   
       public boolean wasNull() throws SQLException {
           return this.wasNullFlag;
       }
   ```
   The default value of `wasNullFlag` is `false`.
   
   PostgreSQL JDBC implementation of `wasNull` (postgresql:42.4.1):
   PgResultSet.java
   ```
     /**
      * True if the last obtained column value was SQL NULL as specified by {@link #wasNull}. The value
      * is always updated by the {@link #getRawValue} method.
      */
     protected boolean wasNullFlag = false;
   
     public boolean wasNull() throws SQLException {
       checkClosed();
       return wasNullFlag;
     }
   ```
   The default value of `wasNullFlag` is `false`.
   
   So when `ResultSet.hasNext()` return `false` (Invoke ResultSet.getXxx() method will throw exception, `wasNullFlag` won't be set with value), `wasNull` should not be invoked, else it might return old value of `wasNullFlag` or default value `false` (if there's no any record in ResultSet).
   
   But I didn't do real test on JDBC driver.
   


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