归并引擎就是将结果和并的引擎，我们先从入口类开始MergeEngine

1.先看下他的属性有哪些

  //数据类型，mysql
  private final DatabaseType databaseType;
  //数据库的一些信息对象
  private final ShardingSphereSchema schema;
  //这是是配置的一些属性
  private final ConfigurationProperties props;

2.将QueryResult根据不同设置进行合并

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)));
    }

3.执行合并的方法，返回合并结果

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(databaseType, entry.getKey(), props, sqlStatementContext);
                return Optional.of(resultMerger.merge(queryResults, sqlStatementContext, schema));
            }
        }
        return Optional.empty();
    }

4.对合并的结果进行再处理

private MergedResult decorate(final MergedResult mergedResult, final SQLStatementContext<?> sqlStatementContext) throws SQLException {
        MergedResult result = null;
        for (Entry<ShardingSphereRule, ResultProcessEngine> entry : engines.entrySet()) {
            if (entry.getValue() instanceof ResultDecoratorEngine) {
                ResultDecorator resultDecorator = ((ResultDecoratorEngine) entry.getValue()).newInstance(databaseType, schema, entry.getKey(), props, sqlStatementContext);
                result = null == result ? resultDecorator.decorate(mergedResult, sqlStatementContext, entry.getKey()) : resultDecorator.decorate(result, sqlStatementContext, entry.getKey());
            }
        }
        return null == result ? mergedResult : result;
    }

5.ShardingDQLResultMerger类，根据不同的数据库类型进行相应的处理

private MergedResult decoratePaginationContext(final List<QueryResult> queryResults, final SelectStatementContext selectStatementContext, final MergedResult mergedResult) throws SQLException {
        PaginationContext paginationContext = selectStatementContext.getPaginationContext();
        if (!paginationContext.isHasPagination() || 1 == queryResults.size()) {
            return mergedResult;
        }
        String trunkDatabaseName = DatabaseTypeRegistry.getTrunkDatabaseType(databaseType.getName()).getName();
        if ("MySQL".equals(trunkDatabaseName) || "PostgreSQL".equals(trunkDatabaseName)) {
            return new LimitDecoratorMergedResult(mergedResult, paginationContext);
        }
        if ("Oracle".equals(trunkDatabaseName)) {
            return new RowNumberDecoratorMergedResult(mergedResult, paginationContext);
        }
        if ("SQLServer".equals(trunkDatabaseName)) {
            return new TopAndRowNumberDecoratorMergedResult(mergedResult, paginationContext);
        }
        return mergedResult;
    }

6.LimitDecoratorMergedResult类和DecoratorMergedResult类，对限定条件进行合并和筛选

public LimitDecoratorMergedResult(final MergedResult mergedResult, final PaginationContext pagination) throws SQLException {
        super(mergedResult);
        this.pagination = pagination;
        skipAll = skipOffset();
    }