Is it possible and worthy to optimize scanRTEForColumn()?

[Rui Hai Jiang]

Hello,

When I run a select query, e.g. select id from t,  all columns in table "t" are 
checked to see if a column named "id" exists or not, and a Var is created for 
"id" if the column does exist.

Function scanRTEForColumn() does this job.

But I see in scanRTEForColumn(), the loop does not stop when a match is found, 
it continues to compare all other columns. And this will waste lots of 
computing.


I guess there may be some reasons for this. But I don't know yet.  I just 
wonder if it is possible and worthy to optimize this. And please note, "select 
*" does not call this function.


Node *
scanRTEForColumn(ParseState *pstate, RangeTblEntry *rte, char *colname,
                                 int location, int fuzzy_rte_penalty,
                                 FuzzyAttrMatchState *fuzzystate)
{
      foreach(c, rte->eref->colnames)
        {
                const char *attcolname = strVal(lfirst(c));
                attnum++;
                if (strcmp(attcolname, colname) == 0)
                {
                         if (result)
                                ereport(ERROR,
                                                
(errcode(ERRCODE_AMBIGUOUS_COLUMN),
                                                 errmsg("column reference 
\"%s\" is ambiguous",
                                                                colname),
                                                 parser_errposition(pstate, 
location)));
                        var = make_var(pstate, rte, attnum, location);
                        /* Require read access to the column */
                        markVarForSelectPriv(pstate, var, rte);
                        result = (Node *) var;
                }

                /* Updating fuzzy match state, if provided. */
                if (fuzzystate != NULL)
                        updateFuzzyAttrMatchState(fuzzy_rte_penalty, fuzzystate,
                                                                          rte, 
attcolname, colname, attnum);
        }

        /*
         * If we have a unique match, return it.  Note that this allows a user
         * alias to override a system column name (such as OID) without error.
         */
        if (result)
                return result;

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}