【源码解析】分库分表框架 Shardingsphere 源码解析

news2024/10/5 14:15:30

前言

以前研究过如何使用ShardingJdbc，使用ShardingJdbc进行分库分表，但是原理方面没有细致的深入了解。如果仅仅了解如何使用的话，对于改造和排查问题，其实都是不够的，所以跟踪源码了解其运行原理是很重要的。

Demo

依赖

        <!-- 分库分表  -->
        <dependency>
            <groupId>org.apache.shardingsphere</groupId>
            <artifactId>shardingsphere-jdbc-core-spring-boot-starter</artifactId>
            <version>5.1.2</version>
        </dependency>

配置文件

spring:
  shardingsphere:
    mode:
      type: Memory                                         # 内存模式，元数据保存在当前进程中
    datasource:
      names: test0,test1                                   # 数据源名称，这里有两个
      test0:                                               # 跟上面的数据源对应
        type: com.alibaba.druid.pool.DruidDataSource       # 连接池
        url: jdbc:mysql://127.0.0.1:3306/test0             # 连接url
        username: root
        password: root
      test1:                                               # 跟上面的数据源对应
        type: com.alibaba.druid.pool.DruidDataSource
        url: jdbc:mysql://127.0.0.1:3306/test1
        username: root
        password: root
    rules:
      sharding:
        tables:
          user:                                            # 这个可以随便取，问题不大
            actual-data-nodes: test$->{0..1}.user$->{0..2} # 实际节点名称，格式为 库名$->{0..n1}.表名$->{0..n2}
              # 其中n1、n2分别为库数量-1和表数量-1
            # 也可以使用${0..n1}的形式，但是会与Spring属性文件占位符冲突
            # 所以使用$->{0..n1}的形式

            database-strategy:                             # 分库策略
              standard:                                    # 标准分库策略
                sharding-column: age                       # 分库列名
                sharding-algorithm-name: age-mod           # 分库算法名字
            table-strategy:                                # 分表策略
              standard:                                    # 标准分表策略
                sharding-column: id                        # 分表列名
                sharding-algorithm-name: id-mod            # 分表算法名字
        sharding-algorithms:                               # 配置分库和分表的算法
          age-mod:                                         # 分库算法名字
            type: MOD                                      # 算法类型为取模
            props:                                         # 算法配置的键名，所有算法配置都需要在props下
              sharding-count: 2                            # 分片数量
          id-mod:                                          # 分表算法名字
            type: MOD                                      # 算法类型为取模
            props:                                         # 算法配置的键名，所有算法配置都需要在props下
              sharding-count: 3                            # 分片数量
    props:
      sql-show: true                                       # 打印SQL

创表语句

-- auto-generated definition
create table user
(
    id   bigint       not null
        primary key,
    name varchar(200) null,
    age  int          null
);

实体类

@Data
@Builder
public class User {
    
    @TableId(type = IdType.ASSIGN_ID)
    private Long id;
    private String name;
    private Integer age;
}

Mapper

public interface UserMapper extends BaseMapper<User> {

}

控制器

@RestController
@RequestMapping("/user")
public class UserController {

    @Autowired
    private UserMapper userMapper;

    @GetMapping("/insert")
    public boolean insert() {
        userMapper.insert(User.builder().name("name").age(new Random().nextInt(100) + 1).build());
        return true;
    }

    @GetMapping("/select")
    public List<User> select() {
        return userMapper.selectList(new QueryWrapper<>());
    }
}

源码解析

解析

PreparedStatementHandler#instantiateStatement，创建Statement会用到sql解析。

    protected Statement instantiateStatement(Connection connection) throws SQLException {
        String sql = this.boundSql.getSql();
        if (this.mappedStatement.getKeyGenerator() instanceof Jdbc3KeyGenerator) {
            String[] keyColumnNames = this.mappedStatement.getKeyColumns();
            return keyColumnNames == null ? connection.prepareStatement(sql, 1) : connection.prepareStatement(sql, keyColumnNames);
        } else {
            return this.mappedStatement.getResultSetType() == ResultSetType.DEFAULT ? connection.prepareStatement(sql) : connection.prepareStatement(sql, this.mappedStatement.getResultSetType().getValue(), 1007);
        }
    }

ShardingSpherePreparedStatement

    public ShardingSpherePreparedStatement(final ShardingSphereConnection connection, final String sql) throws SQLException {
        this(connection, sql, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY, ResultSet.HOLD_CURSORS_OVER_COMMIT, false);
    }

    private ShardingSpherePreparedStatement(final ShardingSphereConnection connection, final String sql,
                                            final int resultSetType, final int resultSetConcurrency, final int resultSetHoldability, final boolean returnGeneratedKeys) throws SQLException {
        if (Strings.isNullOrEmpty(sql)) {
            SQLExceptionErrorCode errorCode = SQLExceptionErrorCode.SQL_STRING_NULL_OR_EMPTY;
            throw new SQLException(errorCode.getErrorMessage(), errorCode.getSqlState(), errorCode.getErrorCode());
        }
        this.connection = connection;
        metaDataContexts = connection.getContextManager().getMetaDataContexts();
        this.sql = sql;
        statements = new ArrayList<>();
        parameterSets = new ArrayList<>();
        Optional<SQLParserRule> sqlParserRule = metaDataContexts.getMetaData().getGlobalRuleMetaData().findSingleRule(SQLParserRule.class);
        Preconditions.checkState(sqlParserRule.isPresent());
        ShardingSphereSQLParserEngine sqlParserEngine = sqlParserRule.get().getSQLParserEngine(
                DatabaseTypeEngine.getTrunkDatabaseTypeName(metaDataContexts.getMetaData().getDatabases().get(connection.getDatabaseName()).getResource().getDatabaseType()));
        sqlStatement = sqlParserEngine.parse(sql, true);
        sqlStatementContext = SQLStatementContextFactory.newInstance(metaDataContexts.getMetaData().getDatabases(), sqlStatement, connection.getDatabaseName());
        parameterMetaData = new ShardingSphereParameterMetaData(sqlStatement);
        statementOption = returnGeneratedKeys ? new StatementOption(true) : new StatementOption(resultSetType, resultSetConcurrency, resultSetHoldability);
        executor = new DriverExecutor(connection);
        JDBCExecutor jdbcExecutor = new JDBCExecutor(connection.getContextManager().getExecutorEngine(), connection.isHoldTransaction());
        batchPreparedStatementExecutor = new BatchPreparedStatementExecutor(metaDataContexts, jdbcExecutor, connection.getDatabaseName());
        kernelProcessor = new KernelProcessor();
        statementsCacheable = isStatementsCacheable(metaDataContexts.getMetaData().getDatabases().get(connection.getDatabaseName()).getRuleMetaData().getConfigurations());
        trafficRule = metaDataContexts.getMetaData().getGlobalRuleMetaData().findSingleRule(TrafficRule.class).orElse(null);
        statementManager = new StatementManager();
    }

SQLStatementParserEngine#parse，判断是否有缓存。

    public SQLStatement parse(String sql, boolean useCache) {
        return useCache ? (SQLStatement)this.sqlStatementCache.get(sql) : this.sqlStatementParserExecutor.parse(sql);
    }

SQLParserExecutor#parse，解析sql，处理成多个片段。

    public ParseASTNode parse(String sql) {
        ParseASTNode result = this.twoPhaseParse(sql);
        if (result.getRootNode() instanceof ErrorNode) {
            throw new SQLParsingException("Unsupported SQL of `%s`", new Object[]{sql});
        } else {
            return result;
        }
    }

路由

PreparedStatementHandler#query，Mybatis执行查询获取到的PreparedStatement是ShardingSpherePreparedStatement。

	public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
        PreparedStatement ps = (PreparedStatement)statement;
        ps.execute();
        return this.resultSetHandler.handleResultSets(ps);
    }

ShardingSpherePreparedStatement#execute，获取逻辑SQL，调试获取到的sql是INSERT INTO user ( id,name,age ) VALUES ( ?,?,? )。

    @Override
    public boolean execute() throws SQLException {
        try {
            if (statementsCacheable && !statements.isEmpty()) {
                resetParameters();
                return statements.iterator().next().execute();
            }
            clearPrevious();
            LogicSQL logicSQL = createLogicSQL();
            trafficContext = getTrafficContext(logicSQL);
            if (trafficContext.isMatchTraffic()) {
                JDBCExecutionUnit executionUnit = createTrafficExecutionUnit(trafficContext, logicSQL);
                return executor.getTrafficExecutor().execute(executionUnit, (statement, sql) -> ((PreparedStatement) statement).execute());
            }
            executionContext = createExecutionContext(logicSQL);
            if (hasRawExecutionRule()) {
                // TODO process getStatement
                Collection<ExecuteResult> executeResults = executor.getRawExecutor().execute(createRawExecutionGroupContext(), executionContext.getLogicSQL(), new RawSQLExecutorCallback());
                return executeResults.iterator().next() instanceof QueryResult;
            }
            if (executionContext.getRouteContext().isFederated()) {
                ResultSet resultSet = executeFederationQuery(logicSQL);
                return null != resultSet;
            }
            ExecutionGroupContext<JDBCExecutionUnit> executionGroupContext = createExecutionGroupContext();
            cacheStatements(executionGroupContext.getInputGroups());
            return executor.getRegularExecutor().execute(executionGroupContext,
                    executionContext.getLogicSQL(), executionContext.getRouteContext().getRouteUnits(), createExecuteCallback());
        } catch (SQLException ex) {
            handleExceptionInTransaction(connection, metaDataContexts);
            throw ex;
        } finally {
            clearBatch();
        }
    }

KernelProcessor#generateExecutionContext，处理sql主要是进行路由，改写

    public ExecutionContext generateExecutionContext(LogicSQL logicSQL, ShardingSphereDatabase database, ConfigurationProperties props) {
        RouteContext routeContext = this.route(logicSQL, database, props);
        SQLRewriteResult rewriteResult = this.rewrite(logicSQL, database, props, routeContext);
        ExecutionContext result = this.createExecutionContext(logicSQL, database, routeContext, rewriteResult);
        this.logSQL(logicSQL, props, result);
        return result;
    }

SQLRouteEngine#route，SQLRouteEngine调用SQLRouteExecutor进行路由，获取到的是PartialSQLRouteExecutor。

    public RouteContext route(final LogicSQL logicSQL, final ShardingSphereDatabase database) {
        SQLRouteExecutor executor = isNeedAllSchemas(logicSQL.getSqlStatementContext().getSqlStatement()) ? new AllSQLRouteExecutor() : new PartialSQLRouteExecutor(rules, props);
        return executor.route(logicSQL, database);
    }

PartialSQLRouteExecutor，根据SQLRouterFactory获取routers。

    public PartialSQLRouteExecutor(final Collection<ShardingSphereRule> rules, final ConfigurationProperties props) {
        this.props = props;
        routers = SQLRouterFactory.getInstances(rules);
    }

SQLRouterFactory#getInstances，通过SPI的方式获取到SQLRouter。

    public static Map<ShardingSphereRule, SQLRouter> getInstances(final Collection<ShardingSphereRule> rules) {
        return OrderedSPIRegistry.getRegisteredServices(SQLRouter.class, rules);
    }

PartialSQLRouteExecutor#route，根据ShardingSphereRule和SQLRouter获取结果。

    public RouteContext route(final LogicSQL logicSQL, final ShardingSphereDatabase database) {
        RouteContext result = new RouteContext();
        Optional<String> dataSourceName = findDataSourceByHint(logicSQL.getSqlStatementContext(), database.getResource().getDataSources());
        if (dataSourceName.isPresent()) {
            result.getRouteUnits().add(new RouteUnit(new RouteMapper(dataSourceName.get(), dataSourceName.get()), Collections.emptyList()));
            return result;
        }
        for (Entry<ShardingSphereRule, SQLRouter> entry : routers.entrySet()) {
            if (result.getRouteUnits().isEmpty()) {
                result = entry.getValue().createRouteContext(logicSQL, database, entry.getKey(), props);
            } else {
                entry.getValue().decorateRouteContext(result, logicSQL, database, entry.getKey(), props);
            }
        }
        if (result.getRouteUnits().isEmpty() && 1 == database.getResource().getDataSources().size()) {
            String singleDataSourceName = database.getResource().getDataSources().keySet().iterator().next();
            result.getRouteUnits().add(new RouteUnit(new RouteMapper(singleDataSourceName, singleDataSourceName), Collections.emptyList()));
        }
        return result;
    }

ShardingSQLRouter#createRouteContext，根据对应的分库分表规则计算结果。

    public RouteContext createRouteContext(LogicSQL logicSQL, ShardingSphereDatabase database, ShardingRule rule, ConfigurationProperties props) {
        SQLStatement sqlStatement = logicSQL.getSqlStatementContext().getSqlStatement();
        ShardingConditions shardingConditions = this.createShardingConditions(logicSQL, database, rule);
        Optional<ShardingStatementValidator> validator = ShardingStatementValidatorFactory.newInstance(sqlStatement, shardingConditions);
        validator.ifPresent((optional) -> {
            optional.preValidate(rule, logicSQL.getSqlStatementContext(), logicSQL.getParameters(), database);
        });
        if (sqlStatement instanceof DMLStatement && shardingConditions.isNeedMerge()) {
            shardingConditions.merge();
        }

        RouteContext result = ShardingRouteEngineFactory.newInstance(rule, database, logicSQL.getSqlStatementContext(), shardingConditions, props).route(rule);
        validator.ifPresent((optional) -> {
            optional.postValidate(rule, logicSQL.getSqlStatementContext(), logicSQL.getParameters(), database, props, result);
        });
        return result;
    }

ShardingStandardRoutingEngine#route，获取到DataNode的集合。

    public RouteContext route(ShardingRule shardingRule) {
        RouteContext result = new RouteContext();
        Collection<DataNode> dataNodes = this.getDataNodes(shardingRule, shardingRule.getTableRule(this.logicTableName));
        result.getOriginalDataNodes().addAll(this.originalDataNodes);
        Iterator var4 = dataNodes.iterator();

        while(var4.hasNext()) {
            DataNode each = (DataNode)var4.next();
            result.getRouteUnits().add(new RouteUnit(new RouteMapper(each.getDataSourceName(), each.getDataSourceName()), Collections.singleton(new RouteMapper(this.logicTableName, each.getTableName()))));
        }

        return result;
    }

ShardingStandardRoutingEngine#getDataNodes，根据shardingCondition的数据进行路由，根据分库策略获取库名，再根据库名和分表策略获取表名。

    private Collection<DataNode> getDataNodes(final ShardingRule shardingRule, final TableRule tableRule) {
        ShardingStrategy databaseShardingStrategy = createShardingStrategy(shardingRule.getDatabaseShardingStrategyConfiguration(tableRule),
                shardingRule.getShardingAlgorithms(), shardingRule.getDefaultShardingColumn());
        ShardingStrategy tableShardingStrategy = createShardingStrategy(shardingRule.getTableShardingStrategyConfiguration(tableRule),
                shardingRule.getShardingAlgorithms(), shardingRule.getDefaultShardingColumn());
        if (isRoutingByHint(shardingRule, tableRule)) {
            return routeByHint(tableRule, databaseShardingStrategy, tableShardingStrategy);
        }
        if (isRoutingByShardingConditions(shardingRule, tableRule)) {
            return routeByShardingConditions(shardingRule, tableRule, databaseShardingStrategy, tableShardingStrategy);
        }
        return routeByMixedConditions(shardingRule, tableRule, databaseShardingStrategy, tableShardingStrategy);
    }

    private Collection<DataNode> routeByShardingConditions(final ShardingRule shardingRule, final TableRule tableRule,
                                                           final ShardingStrategy databaseShardingStrategy, final ShardingStrategy tableShardingStrategy) {
        return shardingConditions.getConditions().isEmpty()
                ? route0(tableRule, databaseShardingStrategy, Collections.emptyList(), tableShardingStrategy, Collections.emptyList())
                : routeByShardingConditionsWithCondition(shardingRule, tableRule, databaseShardingStrategy, tableShardingStrategy);
    }

    private Collection<DataNode> routeByShardingConditionsWithCondition(final ShardingRule shardingRule, final TableRule tableRule,
                                                                        final ShardingStrategy databaseShardingStrategy, final ShardingStrategy tableShardingStrategy) {
        Collection<DataNode> result = new LinkedList<>();
        for (ShardingCondition each : shardingConditions.getConditions()) {
            Collection<DataNode> dataNodes = route0(tableRule,
                    databaseShardingStrategy, getShardingValuesFromShardingConditions(shardingRule, databaseShardingStrategy.getShardingColumns(), each),
                    tableShardingStrategy, getShardingValuesFromShardingConditions(shardingRule, tableShardingStrategy.getShardingColumns(), each));
            result.addAll(dataNodes);
            originalDataNodes.add(dataNodes);
        }
        return result;
    }

    private Collection<DataNode> route0(final TableRule tableRule,
                                        final ShardingStrategy databaseShardingStrategy, final List<ShardingConditionValue> databaseShardingValues,
                                        final ShardingStrategy tableShardingStrategy, final List<ShardingConditionValue> tableShardingValues) {
        Collection<String> routedDataSources = routeDataSources(tableRule, databaseShardingStrategy, databaseShardingValues);
        Collection<DataNode> result = new LinkedList<>();
        for (String each : routedDataSources) {
            result.addAll(routeTables(tableRule, each, tableShardingStrategy, tableShardingValues));
        }
        return result;
    }

ShardingStandardRoutingEngine#routeDataSources

    private Collection<String> routeDataSources(final TableRule tableRule, final ShardingStrategy databaseShardingStrategy, final List<ShardingConditionValue> databaseShardingValues) {
        if (databaseShardingValues.isEmpty()) {
            return tableRule.getActualDatasourceNames();
        }
        Collection<String> result = databaseShardingStrategy.doSharding(tableRule.getActualDatasourceNames(), databaseShardingValues, tableRule.getDataSourceDataNode(), properties);
        Preconditions.checkState(!result.isEmpty(), "No database route info");
        Preconditions.checkState(tableRule.getActualDatasourceNames().containsAll(result),
                "Some routed data sources do not belong to configured data sources. routed data sources: `%s`, configured data sources: `%s`", result, tableRule.getActualDatasourceNames());
        return result;
    }

StandardShardingStrategy#doSharding()

    @Override
    public Collection<String> doSharding(final Collection<String> availableTargetNames, final Collection<ShardingConditionValue> shardingConditionValues,
                                         final DataNodeInfo dataNodeInfo, final ConfigurationProperties props) {
        ShardingConditionValue shardingConditionValue = shardingConditionValues.iterator().next();
        Collection<String> shardingResult = shardingConditionValue instanceof ListShardingConditionValue
                ? doSharding(availableTargetNames, (ListShardingConditionValue) shardingConditionValue, dataNodeInfo)
                : doSharding(availableTargetNames, (RangeShardingConditionValue) shardingConditionValue, dataNodeInfo);
        Collection<String> result = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
        result.addAll(shardingResult);
        return result;
    }

    private Collection<String> doSharding(final Collection<String> availableTargetNames, final ListShardingConditionValue<?> shardingValue, final DataNodeInfo dataNodeInfo) {
        Collection<String> result = new LinkedList<>();
        for (Comparable<?> each : shardingValue.getValues()) {
            String target = shardingAlgorithm.doSharding(availableTargetNames,
                    new PreciseShardingValue(shardingValue.getTableName(), shardingValue.getColumnName(), dataNodeInfo, each));
            if (null != target && availableTargetNames.contains(target)) {
                result.add(target);
            } else if (null != target && !availableTargetNames.contains(target)) {
                throw new ShardingSphereException(String.format("Route table %s does not exist, available actual table: %s", target, availableTargetNames));
            }
        }
        return result;
    }

ModShardingAlgorithm#doSharding()，如果是取余的算法就是获取后缀值。

    @Override
    public String doSharding(final Collection<String> availableTargetNames, final PreciseShardingValue<Comparable<?>> shardingValue) {
        String shardingResultSuffix = getShardingResultSuffix(cutShardingValue(shardingValue.getValue()).mod(new BigInteger(String.valueOf(shardingCount))).toString());
        return findMatchedTargetName(availableTargetNames, shardingResultSuffix, shardingValue.getDataNodeInfo()).orElse(null);
    }

重写

KernelProcessor#rewrite，调用SQLRewriteEntry重写Sql。

    private SQLRewriteResult rewrite(LogicSQL logicSQL, ShardingSphereDatabase database, ConfigurationProperties props, RouteContext routeContext) {
        SQLRewriteEntry sqlRewriteEntry = new SQLRewriteEntry(database, props);
        return sqlRewriteEntry.rewrite(logicSQL.getSql(), logicSQL.getParameters(), logicSQL.getSqlStatementContext(), routeContext);
    }

SQLRewriteEntry#rewrite，调用RouteSQLRewriteEngine重写Sql。

    public SQLRewriteResult rewrite(final String sql, final List<Object> parameters, final SQLStatementContext<?> sqlStatementContext, final RouteContext routeContext) {
        SQLRewriteContext sqlRewriteContext = createSQLRewriteContext(sql, parameters, sqlStatementContext, routeContext);
        SQLTranslatorRule rule = database.getRuleMetaData().findSingleRule(SQLTranslatorRule.class).orElseGet(() -> new SQLTranslatorRule(new SQLTranslatorRuleConfiguration()));
        DatabaseType protocolType = database.getProtocolType();
        DatabaseType storageType = database.getResource().getDatabaseType();
        return routeContext.getRouteUnits().isEmpty()
                ? new GenericSQLRewriteEngine(rule, protocolType, storageType).rewrite(sqlRewriteContext)
                : new RouteSQLRewriteEngine(rule, protocolType, storageType).rewrite(sqlRewriteContext, routeContext);
    }

RouteSQLRewriteEngine#rewrite，重写sql。

    public RouteSQLRewriteResult rewrite(final SQLRewriteContext sqlRewriteContext, final RouteContext routeContext) {
        Map<RouteUnit, SQLRewriteUnit> sqlRewriteUnits = new LinkedHashMap<>(routeContext.getRouteUnits().size(), 1);
        for (Entry<String, Collection<RouteUnit>> entry : aggregateRouteUnitGroups(routeContext.getRouteUnits()).entrySet()) {
            Collection<RouteUnit> routeUnits = entry.getValue();
            if (isNeedAggregateRewrite(sqlRewriteContext.getSqlStatementContext(), routeUnits)) {
                sqlRewriteUnits.put(routeUnits.iterator().next(), createSQLRewriteUnit(sqlRewriteContext, routeContext, routeUnits));
            } else {
                addSQLRewriteUnits(sqlRewriteUnits, sqlRewriteContext, routeContext, routeUnits);
            }
        }
        return new RouteSQLRewriteResult(translate(sqlRewriteContext.getSqlStatementContext().getSqlStatement(), sqlRewriteUnits));
    }

    private void addSQLRewriteUnits(final Map<RouteUnit, SQLRewriteUnit> sqlRewriteUnits, final SQLRewriteContext sqlRewriteContext,
                                    final RouteContext routeContext, final Collection<RouteUnit> routeUnits) {
        for (RouteUnit each : routeUnits) {
            sqlRewriteUnits.put(each, new SQLRewriteUnit(new RouteSQLBuilder(sqlRewriteContext, each).toSQL(), getParameters(sqlRewriteContext.getParameterBuilder(), routeContext, each)));
        }
    }

AbstractSQLBuilder#toSQL，重新生成SQL。

    @Override
    public final String toSQL() {
        if (context.getSqlTokens().isEmpty()) {
            return context.getSql();
        }
        Collections.sort(context.getSqlTokens());
        StringBuilder result = new StringBuilder();
        result.append(context.getSql(), 0, context.getSqlTokens().get(0).getStartIndex());
        for (SQLToken each : context.getSqlTokens()) {
            result.append(each instanceof ComposableSQLToken ? getComposableSQLTokenText((ComposableSQLToken) each) : getSQLTokenText(each));
            result.append(getConjunctionText(each));
        }
        return result.toString();
    }

归并

ShardingSpherePreparedStatement#getResultSet()，归并查询到的结果集。

    @Override
    public ResultSet getResultSet() throws SQLException {
        if (null != currentResultSet) {
            return currentResultSet;
        }
        if (trafficContext.isMatchTraffic()) {
            return executor.getTrafficExecutor().getResultSet();
        }
        if (executionContext.getRouteContext().isFederated()) {
            return executor.getFederationExecutor().getResultSet();
        }
        if (executionContext.getSqlStatementContext() instanceof SelectStatementContext || executionContext.getSqlStatementContext().getSqlStatement() instanceof DALStatement) {
            List<ResultSet> resultSets = getResultSets();
            MergedResult mergedResult = mergeQuery(getQueryResults(resultSets));
            currentResultSet = new ShardingSphereResultSet(resultSets, mergedResult, this, executionContext);
        }
        return currentResultSet;
    }

MergeEngine#merge，执行merge。

    public MergedResult merge(final List<QueryResult> queryResults, final SQLStatementContext<?> sqlStatementContext) throws SQLException {
        Optional<MergedResult> mergedResult = executeMerge(queryResults, sqlStatementContext);
        Optional<MergedResult> result = mergedResult.isPresent() ? Optional.of(decorate(mergedResult.get(), sqlStatementContext)) : decorate(queryResults.get(0), sqlStatementContext);
        return result.orElseGet(() -> new TransparentMergedResult(queryResults.get(0)));
    }
    
    @SuppressWarnings({"unchecked", "rawtypes"})
    private Optional<MergedResult> executeMerge(final List<QueryResult> queryResults, final SQLStatementContext<?> sqlStatementContext) throws SQLException {
        for (Entry<ShardingSphereRule, ResultProcessEngine> entry : engines.entrySet()) {
            if (entry.getValue() instanceof ResultMergerEngine) {
                ResultMerger resultMerger = ((ResultMergerEngine) entry.getValue()).newInstance(
                        database.getName(), database.getResource().getDatabaseType(), entry.getKey(), props, sqlStatementContext);
                return Optional.of(resultMerger.merge(queryResults, sqlStatementContext, database));
            }
        }
        return Optional.empty();
    }

ShardingDQLResultMerger#merge，根据group by、distinct、order by等关键字做不同的归并处理

    public MergedResult merge(final List<QueryResult> queryResults, final SQLStatementContext<?> sqlStatementContext, final ShardingSphereDatabase database) throws SQLException {
        if (1 == queryResults.size() && !isNeedAggregateRewrite(sqlStatementContext)) {
            return new IteratorStreamMergedResult(queryResults);
        }
        Map<String, Integer> columnLabelIndexMap = getColumnLabelIndexMap(queryResults.get(0));
        SelectStatementContext selectStatementContext = (SelectStatementContext) sqlStatementContext;
        selectStatementContext.setIndexes(columnLabelIndexMap);
        MergedResult mergedResult = build(queryResults, selectStatementContext, columnLabelIndexMap, database);
        return decorate(queryResults, selectStatementContext, mergedResult);
    }

    private MergedResult build(final List<QueryResult> queryResults, final SelectStatementContext selectStatementContext,
                               final Map<String, Integer> columnLabelIndexMap, final ShardingSphereDatabase database) throws SQLException {
        String defaultSchemaName = DatabaseTypeEngine.getDefaultSchemaName(selectStatementContext.getDatabaseType(), database.getName());
        ShardingSphereSchema schema = selectStatementContext.getTablesContext().getSchemaName()
                .map(optional -> database.getSchemas().get(optional)).orElseGet(() -> database.getSchemas().get(defaultSchemaName));
        if (isNeedProcessGroupBy(selectStatementContext)) {
            return getGroupByMergedResult(queryResults, selectStatementContext, columnLabelIndexMap, schema);
        }
        if (isNeedProcessDistinctRow(selectStatementContext)) {
            setGroupByForDistinctRow(selectStatementContext);
            return getGroupByMergedResult(queryResults, selectStatementContext, columnLabelIndexMap, schema);
        }
        if (isNeedProcessOrderBy(selectStatementContext)) {
            return new OrderByStreamMergedResult(queryResults, selectStatementContext, schema);
        }
        return new IteratorStreamMergedResult(queryResults);
    }

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