Hive优化器原理与源码解析系列--优化规则SortMergeRule(五)

public boolean matches(RelOptRuleCall call) {
   final HiveSortLimit topSortLimit = call.rel(0); //顶SortLimit
   final HiveSortLimit bottomSortLimit = call.rel(1);//底部SortLimit

   // If top operator is not a pure limit, we bail out
   if (!HiveCalciteUtil.pureLimitRelNode(topSortLimit)) {//判断是否纯limit的RelNode
     return false;
  }
   // If the bottom operator is not synthetic and it does not contain a limit,
   // we will bail out; we do not want to end up with limits all over the tree
   //如果底部操作符不是合成的，并且不包含限制，那么我们将退出；
   if (topSortLimit.isRuleCreated() && !bottomSortLimit.isRuleCreated() &&
           !HiveCalciteUtil.limitRelNode(bottomSortLimit)) {//必须不仅仅包含limit，顶部SortLimit规则创建的，不是底部SortLimit创建的，则优化退出
     return false;
  }
   return true;
}

public void onMatch(RelOptRuleCall call) {
   final HiveSortLimit topSortLimit = call.rel(0);
   final HiveSortLimit bottomSortLimit = call.rel(1);

   final RexNode newOffset;
   final RexNode newLimit;
   if (HiveCalciteUtil.limitRelNode(bottomSortLimit)) {//底部SortLimit
     final RexBuilder rexBuilder = topSortLimit.getCluster().getRexBuilder();
     //分别去顶部和底部SortLimit中 offset fetch
     int topOffset = topSortLimit.offset == null ? 0 : RexLiteral.intValue(topSortLimit.offset);
     int topLimit = RexLiteral.intValue(topSortLimit.fetch);
     int bottomOffset = bottomSortLimit.offset == null ? 0 : RexLiteral.intValue(bottomSortLimit.offset);
     int bottomLimit = RexLiteral.intValue(bottomSortLimit.fetch);
     // Three different cases
     if (topOffset + topLimit <= bottomLimit) {  //如果顶部的offset + fetch 小于 底部的fetch，则新offset = 底部offset + 顶部offset。新fetch = 顶部fetch。取得最小的limit和最大offset
     // 1. Fully contained
     // topOffset + topLimit <= bottomLimit
       newOffset = bottomOffset + topOffset == 0 ? null :
         rexBuilder.makeExactLiteral(BigDecimal.valueOf(bottomOffset + topOffset));
       newLimit = topSortLimit.fetch;
    } else if (topOffset < bottomLimit) {  // 如果 顶部offset 小于 底部fetch，则新offset = 底部offset + 顶部offset，新fetch = 底部fetch - 顶部offset
       // 2. Partially contained
       // topOffset + topLimit > bottomLimit && topOffset < bottomLimit
       newOffset = bottomOffset + topOffset == 0 ? null :
         rexBuilder.makeExactLiteral(BigDecimal.valueOf(bottomOffset + topOffset));
       newLimit = rexBuilder.makeExactLiteral(BigDecimal.valueOf(bottomLimit - topOffset));
    } else { // 其他 新offset = null；新fetch = 0；
       // 3. Outside
       // we need to create a new limit 0
       newOffset = null;
       newLimit = rexBuilder.makeExactLiteral(BigDecimal.valueOf(0));
    }
  } else {
     // Bottom operator does not contain offset/fetch
     newOffset = topSortLimit.offset; //优先底部Sort中的Limit，如果底部不是纯limit，则使用顶部的Sort offset fetch
     newLimit = topSortLimit.fetch;
  }
   final HiveSortLimit newSort = bottomSortLimit.copy(bottomSortLimit.getTraitSet(),
           bottomSortLimit.getInput(), bottomSortLimit.collation, newOffset, newLimit); //使用底部SortLimit，再加上由底部SortLimit和顶部SortLimit的新生成offset 和 fetch合并，新生成的SortLimit注册到优化器去优化器
   call.transformTo(newSort);
}