PostgreSQL数据库事务系统Middle Layer——StartTransactionCommand

StartTransactionCommand函数将事务块状态从TBLOCK_DEFAULT改为TBLOCK_STARTED，并调用底层函数StartTransaction。

void StartTransactionCommand(void) {
  TransactionState s = CurrentTransactionState;
  switch (s->blockState) {
      /* if we aren't in a transaction block, we just do our usual start transaction. */
    case TBLOCK_DEFAULT:
      StartTransaction();
      s->blockState = TBLOCK_STARTED;
      break;
      /* We are somewhere in a transaction block or subtransaction and about to start a new command.  For now we do nothing, but someday we may do command-local resource initialization. (Note that any needed CommandCounterIncrement was done by the previous CommitTransactionCommand.) */
    case TBLOCK_INPROGRESS:
    case TBLOCK_IMPLICIT_INPROGRESS:
    case TBLOCK_SUBINPROGRESS:
      break;
      /* Here we are in a failed transaction block (one of the commands caused an abort) so we do nothing but remain in the abort state.  Eventually we will get a ROLLBACK command which will get us out of this state.  (It is up to other code to ensure that no commands other than ROLLBACK will be processed in these states.) */
    case TBLOCK_ABORT:
    case TBLOCK_SUBABORT:
      break;
      /* These cases are invalid. */
    case TBLOCK_STARTED:
    case TBLOCK_BEGIN:
    case TBLOCK_PARALLEL_INPROGRESS:
    case TBLOCK_SUBBEGIN:
    case TBLOCK_END:
    case TBLOCK_SUBRELEASE:
    case TBLOCK_SUBCOMMIT:
    case TBLOCK_ABORT_END:
    case TBLOCK_SUBABORT_END:
    case TBLOCK_ABORT_PENDING:
    case TBLOCK_SUBABORT_PENDING:
    case TBLOCK_SUBRESTART:
    case TBLOCK_SUBABORT_RESTART:
    case TBLOCK_PREPARE:
      elog(ERROR, "StartTransactionCommand: unexpected state %s", BlockStateAsString(s->blockState));
      break;
  }
  /* We must switch to CurTransactionContext before returning. This is already done if we called StartTransaction, otherwise not. */
  Assert(CurTransactionContext != NULL);
  MemoryContextSwitchTo(CurTransactionContext);
}