Hash Join

HashJoinExec struct

TiDB 源码阅读系列文章（九）Hash Join

HashJoin整体流程如下，主要有三个4个goroutine, 几个goroutine之间通过channel 来协作.

  • fetchBuildSideRows读取buildSides表中数据, 放入buildSideResultCh中
  • fetchAndBuildHashTable 根据buildSide表中数据, 创建hashRowContainer
  • fetchProbeSideChunks: 读取probeSide表数据, 放入probeResultChs中
  • runJoinWorker 多个joinWorker并发执行，从probeResultChs读取probe数据，然后和rowContainer做匹配, 并将结果放入joinResultCh中

hash join

fetchAndBuildHashTable

读取buildSideExec中所有数据，然后写入hasRowContainer中 如果内存不够，会写到磁盘上

fetchProbeSideChunks

fetchProbeSideChunks get chunks from fetches chunks from the big table in a background goroutine and sends the chunks to multiple channels which will be read by multiple join workers.

runJoinWorker

HashJoinExec.Next会启动多个runJoinWorker来做hashJoin,

每个runJoinWorker会从probeResultChs chan中去取要Probe的数据 然后做Join,最后写到joinResultCh中，由HashJoinExec.Next接受， 返回给上层调用者。

join2Chunk

join2Chunk负责将probeResult和inner table做Join