if (va_cols != NIL){
    Bitmapset  *unique_cols = NULL; ListCell   *le; tcnt = 0;
    vacattrstats = (VacAttrStats **) palloc(list_length(va_cols) *sizeof(VacAttrStats *));    
    foreach(le, va_cols){
      char     *col = strVal(lfirst(le));
      i = attnameAttNum(onerel, col, false); 
      if (i == InvalidAttrNumber) ereport(ERROR,(errcode(ERRCODE_UNDEFINED_COLUMN),errmsg("column \"%s\" of relation \"%s\" does not exist",col, RelationGetRelationName(onerel))));
      if (bms_is_member(i, unique_cols)) ereport(ERROR,(errcode(ERRCODE_DUPLICATE_COLUMN),errmsg("column \"%s\" of relation \"%s\" appears more than once",col, RelationGetRelationName(onerel))));
      unique_cols = bms_add_member(unique_cols, i);
      vacattrstats[tcnt] = examine_attribute(onerel, i, NULL);
      if (vacattrstats[tcnt] != NULL) tcnt++;
    }
    attr_cnt = tcnt;
  }

static VacAttrStats *examine_attribute(Relation onerel, int attnum, Node *index_expr){
  Form_pg_attribute attr = TupleDescAttr(onerel->rd_att, attnum - 1);HeapTuple  typtuple;
  VacAttrStats *stats; int      i; bool    ok;


  if (attr->attisdropped) return NULL; * Never analyze dropped columns */  如果该列被drop，则直接返回  
  if (attr->attstattarget == 0) return NULL; * Don't analyze column if user has specified not to */  如果用户指定不要进行分析，则直接返回


  * Create the VacAttrStats struct.  Note that we only have a copy of the fixed fields of the pg_attribute tuple. */
  stats = (VacAttrStats *) palloc0(sizeof(VacAttrStats));
  stats->attr = (Form_pg_attribute) palloc(ATTRIBUTE_FIXED_PART_SIZE);
  memcpy(stats->attr, attr, ATTRIBUTE_FIXED_PART_SIZE);  将pg_attribute系统表中的元组设置到stat->attr中


  /* When analyzing an expression index, believe the expression tree's type not the column datatype --- the latter might be the opckeytype storage type of the opclass, which is not interesting for our purposes.  (Note: if we did anything with non-expression index columns, we'd need to figure out where to get the correct type info from, but for now that's not a problem.)  It's not clear whether anyone will care about the typmod, but we store that too just in case. */ // 在分析表达式索引时，请相信表达式树的类型而不是列数据类型——后者可能是opclass的opckeytype存储类型，这对于我们的目的来说并不有趣。（注意：如果我们对非表达式索引列进行了任何操作，我们需要找出从何处获取正确的类型信息，但目前这不是问题。）不清楚是否有人会关心typmod，但我们也存储它，以防万一。
  if (index_expr){
    stats->attrtypid = exprType(index_expr);
    stats->attrtypmod = exprTypmod(index_expr);
    /* If a collation has been specified for the index column, use that in preference to anything else; but if not, fall back to whatever we can get from the expression. */ // 如果为索引列指定了排序规则，请优先使用该排序规则；但如果不是，我们可以从表达式中得到任何信息
    if (OidIsValid(onerel->rd_indcollation[attnum - 1])) stats->attrcollid = onerel->rd_indcollation[attnum - 1];
    else stats->attrcollid = exprCollation(index_expr);
  }else{
    stats->attrtypid = attr->atttypid;
    stats->attrtypmod = attr->atttypmod;
    stats->attrcollid = attr->attcollation;
  }


  typtuple = SearchSysCacheCopy1(TYPEOID, ObjectIdGetDatum(stats->attrtypid)); // 获取pg_type元组
  if (!HeapTupleIsValid(typtuple)) elog(ERROR, "cache lookup failed for type %u", stats->attrtypid);
  stats->attrtype = (Form_pg_type) GETSTRUCT(typtuple); // 将pg_type系统表中对应元组设置到attrtype中
  stats->anl_context = anl_context;
  stats->tupattnum = attnum;


  /* The fields describing the stats->stavalues[n] element types default to the type of the data being analyzed, but the type-specific typanalyze function can change them if it wants to store something else. */
  for (i = 0; i < STATISTIC_NUM_SLOTS; i++) {
    stats->statypid[i] = stats->attrtypid;
    stats->statyplen[i] = stats->attrtype->typlen;
    stats->statypbyval[i] = stats->attrtype->typbyval;
    stats->statypalign[i] = stats->attrtype->typalign;
  }


  /* Call the type-specific typanalyze function.  If none is specified, use std_typanalyze(). */
  if (OidIsValid(stats->attrtype->typanalyze)) ok = DatumGetBool(OidFunctionCall1(stats->attrtype->typanalyze, PointerGetDatum(stats)));
  else ok = std_typanalyze(stats);
  if (!ok || stats->compute_stats == NULL || stats->minrows <= 0){ heap_freetuple(typtuple);pfree(stats->attr);pfree(stats);return NULL; }
  return stats;
}

bool std_typanalyze(VacAttrStats *stats) {
  Form_pg_attribute attr = stats->attr; // 使用VacAttrStats的attr成员，是pg_attribute元组的拷贝
  Oid      ltopr;  Oid      eqopr;
  StdAnalyzeData *mystats;


  /* If the attstattarget column is negative, use the default value.  NB: it is okay to scribble on stats->attr since it's a copy */ // 如果attstattarget列为负，则使用默认值。注意：可以改变stats->attr，因为它是一个副本
  if (attr->attstattarget < 0) attr->attstattarget = default_statistics_target; // default_statistics_target是一个GUC参数，默认为100  
  get_sort_group_operators(stats->attrtypid, false, false, false, &ltopr, &eqopr, NULL, NULL); /* Look for default "<" and "=" operators for column's type */  
  mystats = (StdAnalyzeData *) palloc(sizeof(StdAnalyzeData)); /* Save the operator info for compute_stats routines */
  mystats->eqopr = eqopr;
  mystats->eqfunc = OidIsValid(eqopr) ? get_opcode(eqopr) : InvalidOid;
  mystats->ltopr = ltopr;
  stats->extra_data = mystats;


  /* Determine which standard statistics algorithm to use */
  if (OidIsValid(eqopr) && OidIsValid(ltopr)) {    
    stats->compute_stats = compute_scalar_stats; /* Seems to be a scalar datatype */
    stats->minrows = 300 * attr->attstattarget;
  }else if (OidIsValid(eqopr)){    
    stats->compute_stats = compute_distinct_stats; /* We can still recognize distinct values */    
    stats->minrows = 300 * attr->attstattarget; /* Might as well use the same minrows as above */
  }else{    
    stats->compute_stats = compute_trivial_stats; /* Can't do much but the trivial stuff */    
    stats->minrows = 300 * attr->attstattarget; /* Might as well use the same minrows as above */
  }


  return true;
}