PG的空值相加如何实现

postgres=# create table t3(id1 int,id2 int);
CREATE TABLE
postgres=# insert into t3 select 1,2;
INSERT 0 1
postgres=# insert into t3 select 1,NULL;
INSERT 0 1
postgres=# insert into t3 select NULL,NULL;
INSERT 0 1
postgres=# insert into t3 select NULL,3;
INSERT 0 1
postgres=# select *from t3;
 id1 | id2
-----+-----
   1 |   2
   1 |
     |
     |   3
(4 行记录)

postgres=# select id1+id2 from t3;
 ?column?
----------
        3



(4 行记录)

ExecInterpExpr
  EEO_CASE(EEOP_FUNCEXPR_STRICT)//操作符函数的执行
  {
    FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
    NullableDatum *args = fcinfo->args;
    int      argno;
    Datum    d;

    /* strict function, so check for NULL args */
    for (argno = 0; argno < op->d.func.nargs; argno++)
    {
      if (args[argno].isnull)
      {
        *op->resnull = true;//只要有一个参数是NULL，就标记resnull为true
        goto strictfail;
      }
    }
    fcinfo->isnull = false;
    d = op->d.func.fn_addr(fcinfo);
    *op->resvalue = d;
    *op->resnull = fcinfo->isnull;

strictfail:
    EEO_NEXT();
  }
  ...
  EEO_CASE(EEOP_ASSIGN_TMP_MAKE_RO)
  {
    int  resultnum = op->d.assign_tmp.resultnum;
    //上一步的resnull值传递到resultslot中，标记该slot是一个NULL
    resultslot->tts_isnull[resultnum] = state->resnull;
    if (!resultslot->tts_isnull[resultnum])
      resultslot->tts_values[resultnum] =
        MakeExpandedObjectReadOnlyInternal(state->resvalue);
    else
      resultslot->tts_values[resultnum] = state->resvalue;
    EEO_NEXT();
  }