Postgresql源码（64）查询执行——子模块Executor（2）执行前的数据结构和执行过程

explain select t.name, c.name, stu_num
from course as c, teach_course as tc, teacher as t
where c.no = tc.cno and tc.tno = t.no and c.name = 'Database System' and t.name = 'Jennifer';
                                    QUERY PLAN                                     
-----------------------------------------------------------------------------------
 Nested Loop  (cost=24.10..74.58 rows=1 width=68)
   ->  Hash Join  (cost=23.95..62.61 rows=61 width=40)
         Hash Cond: (tc.tno = t.no)
         ->  Seq Scan on teach_course tc  (cost=0.00..30.40 rows=2040 width=12)
         ->  Hash  (cost=23.88..23.88 rows=6 width=36)
               ->  Seq Scan on teacher t  (cost=0.00..23.88 rows=6 width=36)
                     Filter: ((name)::text = 'Jennifer'::text)
   ->  Index Scan using course_pkey on course c  (cost=0.15..0.20 rows=1 width=36)
         Index Cond: (no = tc.cno)
         Filter: ((name)::text = 'Database System'::text)

select t.name, c.name, stu_num
from course as c, teach_course as tc, teacher as t
where c.no = tc.cno and tc.tno = t.no and c.name = 'Database System' and t.name = 'Jennifer';

ExecNestLoop
  ...
  outerPlan = outerPlanState(node)    // left node
  innerPlan = innerPlanState(node)    // right node
  // loop until we return a qualifying join tuple
  for (;;)
    // 从outer表的计划上找到一条
    outerTupleSlot = ExecProcNode(outerPlan)
  // 完了去inner表里面拿一条（要能连得上的一条）
  innerTupleSlot = ExecProcNode(innerPlan)
    ExecIndexScan
      for (;;)
        // 【重要】注意这个函数，ExecScanFetch没有具体业务逻辑，这是个框架函数
        // ExecScanFetch用accessMtd拿到元组，在用recheckMtd检查元组是否符合要求
        // 这里用的是两个通用函数 IndexNext 和 IndexRecheck
        slot = ExecScanFetch(node, accessMtd, recheckMtd)
          // 进入bt堆栈
          IndexNext

ExecHashJoinImpl
  ExecHashTableCreate
  // 拿一条左表中的数据
  ExecHashJoinOuterGetTuple
    // T_SeqScanState
    slot = ExecProcNode(outerNode)
  // 找到能连上的tuple所在的bucket
  ExecHashGetBucketAndBatch
  ExecHashGetSkewBucket
  // 查找bucket
  ExecScanHashBucket
  // 找到连接元组

SeqNext
  scandesc = table_beginscan
  table_scan_getnextslot
    slot->tts_tableOid = RelationGetRelid(sscan->rs_rd)
    heap_getnextslot
      heapgettup_pagemode
        heapgetpage
        BufferGetPage
        tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp)