Improving the usage of streams on SQL fetches.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Storage minimum level parameter removed from cylindrical thermal storage [#1123](https://github.com/ie3-institute/PowerSystemDataModel/issues/1123)
 - Converted eval-rst to myst syntax in ReadTheDocs, fixed line wrapping and widths[#1137](https://github.com/ie3-institute/PowerSystemDataModel/issues/1137)
+- Improving usage of streams on sql fetches [#827](https://github.com/ie3-institute/PowerSystemDataModel/issues/827)
 
 ## [5.1.0] - 2024-06-24
 

src/main/java/edu/ie3/datamodel/io/connectors/SqlConnector.java

@@ -9,6 +9,8 @@
 import edu.ie3.util.TimeUtil;
 import java.sql.*;
 import java.util.*;
+import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -55,6 +57,9 @@ public ResultSet executeQuery(Statement stmt, String query) throws SQLException
       return stmt.executeQuery(query);
     } catch (SQLException e) {
       throw new SQLException(String.format("Error at execution of query \"%1.127s\": ", query), e);
+    } finally {
+      // commits any changes made and unlocks database
+      getConnection().commit();
     }
   }
 
@@ -70,6 +75,9 @@ public int executeUpdate(String query) throws SQLException {
     } catch (SQLException e) {
       throw new SQLException(
           String.format("Error at execution of query, SQLReason: '%s'", e.getMessage()), e);
+    } finally {
+      // commits any changes made and unlocks database
+      getConnection().commit();
     }
   }
 
@@ -85,7 +93,8 @@ public Connection getConnection() throws SQLException {
   }
 
   /**
-   * Establishes and returns a database connection
+   * Establishes and returns a database connection. The {@link Connection#getAutoCommit()} is set to
+   * {@code false}.
    *
    * @param reuseConnection should the connection be used again, if it is still valid? If not, a new
    *     connection will be established
@@ -98,6 +107,7 @@ public Connection getConnection(boolean reuseConnection) throws SQLException {
         if (connection != null) connection.close();
 
         connection = DriverManager.getConnection(jdbcUrl, connectionProps);
+        connection.setAutoCommit(false);
       } catch (SQLException e) {
         throw new SQLException("Could not establish connection: ", e);
       }
@@ -115,21 +125,82 @@ public void shutdown() {
   }
 
   /**
-   * Extracts all field to value maps from the ResultSet, one for each row
+   * Method to execute a {@link PreparedStatement} and return its result as a stream.
    *
-   * @param rs the ResultSet to use
-   * @return a list of field maps
+   * @param ps to execute
+   * @param fetchSize used for {@link PreparedStatement#setFetchSize(int)}
+   * @return a stream of maps
+   * @throws SQLException if an exception occurred while executing the query
    */
-  public List<Map<String, String>> extractFieldMaps(ResultSet rs) {
-    List<Map<String, String>> fieldMaps = new ArrayList<>();
+  public Stream<Map<String, String>> toStream(PreparedStatement ps, int fetchSize)
+      throws SQLException {
     try {
-      while (rs.next()) {
-        fieldMaps.add(extractFieldMap(rs));
+      ps.setFetchSize(fetchSize);
+      ResultSet resultSet = ps.executeQuery();
+      Iterator<Map<String, String>> sqlIterator = getSqlIterator(resultSet);
+
+      return StreamSupport.stream(
+              Spliterators.spliteratorUnknownSize(
+                  sqlIterator, Spliterator.NONNULL | Spliterator.IMMUTABLE),
+              true)
+          .onClose(() -> closeResultSet(ps, resultSet));
+    } catch (SQLException e) {
+      // catches the exception, closes the statement and re-throws the exception
+      closeResultSet(ps, null);
+      throw e;
+    }
+  }
+
+  /**
+   * Returns an {@link Iterator} for the given {@link ResultSet}.
+   *
+   * @param rs given result set
+   * @return an iterator
+   */
+  public Iterator<Map<String, String>> getSqlIterator(ResultSet rs) {
+    return new Iterator<>() {
+      @Override
+      public boolean hasNext() {
+        try {
+          return rs.next();
+        } catch (SQLException e) {
+          log.error("Exception at extracting next ResultSet: ", e);
+          closeResultSet(null, rs);
+          return false;
+        }
       }
+
+      @Override
+      public Map<String, String> next() {
+        try {
+          boolean isEmpty = !rs.isBeforeFirst() && rs.getRow() == 0;
+
+          if (isEmpty || rs.isAfterLast())
+            throw new NoSuchElementException(
+                "There is no more element to iterate to in the ResultSet.");
+
+          return extractFieldMap(rs);
+        } catch (SQLException e) {
+          log.error("Exception at extracting ResultSet: ", e);
+          closeResultSet(null, rs);
+          return Collections.emptyMap();
+        }
+      }
+    };
+  }
+
+  /**
+   * Method for closing a {@link ResultSet}.
+   *
+   * @param rs to close
+   */
+  private void closeResultSet(PreparedStatement ps, ResultSet rs) {
+    try (ps;
+        rs) {
+      log.debug("Resources successfully closed.");
     } catch (SQLException e) {
-      log.error("Exception at extracting ResultSet: ", e);
+      log.warn("Failed to properly close sources.", e);
     }
-    return fieldMaps;
   }
 
   /**
@@ -138,26 +209,24 @@ public List<Map<String, String>> extractFieldMaps(ResultSet rs) {
    * @param rs the ResultSet to use
    * @return the field map for the current row
    */
-  public Map<String, String> extractFieldMap(ResultSet rs) {
+  public Map<String, String> extractFieldMap(ResultSet rs) throws SQLException {
     TreeMap<String, String> insensitiveFieldsToAttributes =
         new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
-    try {
-      ResultSetMetaData metaData = rs.getMetaData();
-      int columnCount = metaData.getColumnCount();
-      for (int i = 1; i <= columnCount; i++) {
-        String columnName = StringUtils.snakeCaseToCamelCase(metaData.getColumnName(i));
-        String value;
-        Object result = rs.getObject(i);
-        if (result instanceof Timestamp) {
-          value = TimeUtil.withDefaults.toString(rs.getTimestamp(i).toInstant());
-        } else {
-          value = String.valueOf(rs.getObject(i));
-        }
-        insensitiveFieldsToAttributes.put(columnName, value);
+
+    ResultSetMetaData metaData = rs.getMetaData();
+    int columnCount = metaData.getColumnCount();
+    for (int i = 1; i <= columnCount; i++) {
+      String columnName = StringUtils.snakeCaseToCamelCase(metaData.getColumnName(i));
+      String value;
+      Object result = rs.getObject(i);
+      if (result instanceof Timestamp) {
+        value = TimeUtil.withDefaults.toString(rs.getTimestamp(i).toInstant());
+      } else {
+        value = String.valueOf(rs.getObject(i));
       }
-    } catch (SQLException e) {
-      log.error("Exception at extracting ResultSet: ", e);
+      insensitiveFieldsToAttributes.put(columnName, value);
     }
+
     return insensitiveFieldsToAttributes;
   }
 }

src/main/java/edu/ie3/datamodel/io/source/sql/SqlDataSource.java

@@ -177,10 +177,15 @@ protected Stream<Map<String, String>> buildStreamByTableName(String tableName) {
    * table name.
    */
   protected Stream<Map<String, String>> executeQuery(String query, AddParams addParams) {
-    try (PreparedStatement ps = connector.getConnection().prepareStatement(query)) {
+    try {
+      PreparedStatement ps = connector.getConnection().prepareStatement(query);
       addParams.addParams(ps);
-      ResultSet resultSet = ps.executeQuery();
-      return connector.extractFieldMaps(resultSet).stream();
+
+      // don't work with `try with resource`, therefore manual closing is necessary
+      // closes automatically after all dependent resultSets are closed
+      ps.closeOnCompletion();
+
+      return connector.toStream(ps, 1000);
     } catch (SQLException e) {
       log.error("Error during execution of query {}", query, e);
     }

src/main/java/edu/ie3/datamodel/io/source/sql/SqlIdCoordinateSource.java

@@ -18,8 +18,8 @@
 import edu.ie3.util.geo.CoordinateDistance;
 import edu.ie3.util.geo.GeoUtils;
 import java.sql.Array;
-import java.sql.PreparedStatement;
 import java.util.*;
+import java.util.stream.Stream;
 import javax.measure.quantity.Length;
 import org.locationtech.jts.geom.Envelope;
 import org.locationtech.jts.geom.Point;
@@ -98,7 +98,8 @@ public Optional<Set<String>> getSourceFields() {
 
   @Override
   public Optional<Point> getCoordinate(int id) {
-    List<CoordinateValue> values = executeQueryToList(queryForPoint, ps -> ps.setInt(1, id));
+    List<CoordinateValue> values =
+        executeQueryToStream(queryForPoint, ps -> ps.setInt(1, id)).toList();
 
     if (values.isEmpty()) {
       return Optional.empty();
@@ -111,15 +112,14 @@ public Optional<Point> getCoordinate(int id) {
   public Collection<Point> getCoordinates(int... ids) {
     Object[] idSet = Arrays.stream(ids).boxed().distinct().toArray();
 
-    List<CoordinateValue> values =
-        executeQueryToList(
+    return executeQueryToStream(
             queryForPoints,
             ps -> {
               Array sqlArray = ps.getConnection().createArrayOf("int", idSet);
               ps.setArray(1, sqlArray);
-            });
-
-    return values.stream().map(value -> value.coordinate).toList();
+            })
+        .map(value -> value.coordinate)
+        .toList();
   }
 
   @Override
@@ -128,12 +128,13 @@ public Optional<Integer> getId(Point coordinate) {
     double longitude = coordinate.getX();
 
     List<CoordinateValue> values =
-        executeQueryToList(
-            queryForId,
-            ps -> {
-              ps.setDouble(1, longitude);
-              ps.setDouble(2, latitude);
-            });
+        executeQueryToStream(
+                queryForId,
+                ps -> {
+                  ps.setDouble(1, longitude);
+                  ps.setDouble(2, latitude);
+                })
+            .toList();
 
     if (values.isEmpty()) {
       return Optional.empty();
@@ -144,23 +145,21 @@ public Optional<Integer> getId(Point coordinate) {
 
   @Override
   public Collection<Point> getAllCoordinates() {
-    List<CoordinateValue> values = executeQueryToList(basicQuery + ";", PreparedStatement::execute);
-
-    return values.stream().map(value -> value.coordinate).toList();
+    return executeQueryToStream(basicQuery + ";").map(value -> value.coordinate).toList();
   }
 
   @Override
   public List<CoordinateDistance> getNearestCoordinates(Point coordinate, int n) {
-    List<CoordinateValue> values =
-        executeQueryToList(
-            queryForNearestPoints,
-            ps -> {
-              ps.setDouble(1, coordinate.getX());
-              ps.setDouble(2, coordinate.getY());
-              ps.setInt(3, n);
-            });
-
-    List<Point> points = values.stream().map(value -> value.coordinate).toList();
+    List<Point> points =
+        executeQueryToStream(
+                queryForNearestPoints,
+                ps -> {
+                  ps.setDouble(1, coordinate.getX());
+                  ps.setDouble(2, coordinate.getY());
+                  ps.setInt(3, n);
+                })
+            .map(value -> value.coordinate)
+            .toList();
     return calculateCoordinateDistances(coordinate, n, points);
   }
 
@@ -185,15 +184,14 @@ private List<Point> getCoordinatesInBoundingBox(
       Point coordinate, ComparableQuantity<Length> distance) {
     Envelope envelope = GeoUtils.calculateBoundingBox(coordinate, distance);
 
-    return executeQueryToList(
+    return executeQueryToStream(
             queryForBoundingBox,
             ps -> {
               ps.setDouble(1, envelope.getMinX());
               ps.setDouble(2, envelope.getMinY());
               ps.setDouble(3, envelope.getMaxX());
               ps.setDouble(4, envelope.getMaxY());
             })
-        .stream()
         .map(value -> value.coordinate)
         .toList();
   }
@@ -208,9 +206,13 @@ private CoordinateValue createCoordinateValue(Map<String, String> fieldToValues)
     return new CoordinateValue(idCoordinate.id(), idCoordinate.point());
   }
 
-  private List<CoordinateValue> executeQueryToList(
+  private Stream<CoordinateValue> executeQueryToStream(String query) {
+    return dataSource.executeQuery(query).map(this::createCoordinateValue);
+  }
+
+  private Stream<CoordinateValue> executeQueryToStream(
       String query, SqlDataSource.AddParams addParams) {
-    return dataSource.executeQuery(query, addParams).map(this::createCoordinateValue).toList();
+    return dataSource.executeQuery(query, addParams).map(this::createCoordinateValue);
   }
 
   /**

src/test/groovy/edu/ie3/datamodel/io/connectors/SqlConnectorIT.groovy

@@ -121,16 +121,15 @@ class SqlConnectorIT extends Specification implements TestContainerHelper {
   def "A SQL connector is able to extract all field to value maps from result set"() {
     given:
     def preparedStatement = connector.getConnection(false).prepareStatement("SELECT * FROM public.test;")
-    def resultSet = preparedStatement.executeQuery()
 
     when:
-    def actual = connector.extractFieldMaps(resultSet)
+    def actual = connector.toStream(preparedStatement, 1).toList()
 
     then:
     actual.size() == 2
 
     cleanup:
-    resultSet.close()
+    preparedStatement.close()
   }
 
   def "A SQL connector shuts down correctly, if no connection was opened"() {