PostgreSQL数据库TableAM——HeapAM Scans

static const TableAmRoutine heapam_methods = {
  .type = T_TableAmRoutine,
  .slot_callbacks = heapam_slot_callbacks,
  
  .scan_begin = heap_beginscan,
  .scan_end = heap_endscan,
  .scan_rescan = heap_rescan,
  .scan_getnextslot = heap_getnextslot,
  
  .parallelscan_estimate = table_block_parallelscan_estimate,
  .parallelscan_initialize = table_block_parallelscan_initialize,
  .parallelscan_reinitialize = table_block_parallelscan_reinitialize,
  ...
}

heap_beginscan

heap_beginscan函数用于初始化HeapScanDescData结构体，ParallelTableScanDesc形参在ExecGather函数中ExecSeqScanInitializeDSM函数会调用TableAM提供的table_beginscan_parallel函数，该函数中会申请ParallelTableScanDesc内存，而串行scan时，heap_beginscan函数在SeqNext函数执行时调用，但是ParallelTableScanDesc形参为null。

TableScanDesc heap_beginscan(Relation relation, Snapshot snapshot, int nkeys, ScanKey key,
         ParallelTableScanDesc parallel_scan, uint32 flags) {
  RelationIncrementReferenceCount(relation);
  HeapScanDesc scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));
  scan->rs_base.rs_rd = relation; scan->rs_base.rs_snapshot = snapshot; scan->rs_base.rs_nkeys = nkeys;
  scan->rs_base.rs_flags = flags; scan->rs_base.rs_parallel = parallel_scan; scan->rs_strategy = NULL; /* set in initscan */ 
  if (!(snapshot && IsMVCCSnapshot(snapshot))) scan->rs_base.rs_flags &= ~SO_ALLOW_PAGEMODE; /* Disable page-at-a-time mode if it's not a MVCC-safe snapshot. */
  if (scan->rs_base.rs_flags & (SO_TYPE_SEQSCAN | SO_TYPE_SAMPLESCAN)){
    /* Ensure a missing snapshot is noticed reliably, even if the isolation mode means predicate locking isn't performed (and therefore the snapshot isn't used here). */
    PredicateLockRelation(relation, snapshot);
  } 
  scan->rs_ctup.t_tableOid = RelationGetRelid(relation); /* we only need to set this up once */
  /* we do this here instead of in initscan() because heap_rescan also calls initscan() and we don't want to allocate memory again */
  if (nkeys > 0) scan->rs_base.rs_key = (ScanKey) palloc(sizeof(ScanKeyData) * nkeys);
  else scan->rs_base.rs_key = NULL;

  initscan(scan, key, false);

  return (TableScanDesc) scan;
}

heap_endscan

void heap_endscan(TableScanDesc sscan) {
  HeapScanDesc scan = (HeapScanDesc) sscan;
  /* unpin scan buffers */
  if (BufferIsValid(scan->rs_cbuf)) ReleaseBuffer(scan->rs_cbuf);
  RelationDecrementReferenceCount(scan->rs_base.rs_rd);
  if (scan->rs_base.rs_key) pfree(scan->rs_base.rs_key);
  if (scan->rs_strategy != NULL) FreeAccessStrategy(scan->rs_strategy);
  if (scan->rs_base.rs_flags & SO_TEMP_SNAPSHOT)
    UnregisterSnapshot(scan->rs_base.rs_snapshot);
  pfree(scan);
}

heap_getnextslot

heap_getnext函数获取tuple数据元组，分为两种页扫描模式：如果scan->rs_base.rs_flags为SO_ALLOW_PAGEMODE即扫描所有满足可见性的记录heapgettup_pagemode，否则扫描所有记录heapgettup。

typedef enum ScanDirection {
  BackwardScanDirection = -1,
  NoMovementScanDirection = 0, // re-fetch the tuple indicated by scan->rs_ctup
  ForwardScanDirection = 1
} ScanDirection;

HeapTuple heap_getnext(TableScanDesc sscan, ScanDirection direction) {
  HeapScanDesc scan = (HeapScanDesc) sscan;
  if (unlikely(sscan->rs_rd->rd_tableam != GetHeapamTableAmRoutine()))
    ereport(ERROR,(errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg_internal("only heap AM is supported")));
  if (scan->rs_base.rs_flags & SO_ALLOW_PAGEMODE)
    heapgettup_pagemode(scan, direction, scan->rs_base.rs_nkeys, scan->rs_base.rs_key);
  else
    heapgettup(scan, direction, scan->rs_base.rs_nkeys, scan->rs_base.rs_key);

  if (scan->rs_ctup.t_data == NULL) { return NULL }  // returning EOS

  pgstat_count_heap_getnext(scan->rs_base.rs_rd);
  return &scan->rs_ctup;
}