Thread

Re: [HACKERS] Re: [BUGS] General Bug Report: Bug in optimizer

Bruce Momjian <maillist@candle.pha.pa.us> — 1999-03-18T21:33:11Z

> Bruce Momjian wrote:
> > 
> > Let me tell you why I don't think this is a bug.  The optimizer will
> > choose ordered results over unordered results if the costs are the same.
> > In this case, the cost of the query is zero, so it chose to use the
> > index because the index produces an ordered result.
> > 
> > This works well for un-vacuumed tables, because it thinks everything is
> > zero cost, and chooses the index.
> 
> Agreed, this is ok as long as
> 
> vac=> create table table1 (field1 int);
> CREATE
> vac=> insert into table1 values (1);
> INSERT 1583349 1
> vac=> create index i_table1__field1 on table1 (field1);
> CREATE
> vac=> explain select * from table1 where field1 = 1;
> NOTICE:  QUERY PLAN:
> 
> Seq Scan on table1  (cost=1.03 size=1 width=4)
> 
> - SeqScan is used for small tables.
> 
> So, only bug reported is left.
> 
> Vadim
> 

Fixed:
	
	test=> explain select * from table1 where field1 = 1;
	NOTICE:  QUERY PLAN:
	
	Index Scan using i_table1__field1 on table1  (cost=0.00 size=0 width=4)
	
	EXPLAIN
	test=> explain select * from table1 where field1 = -1;
	NOTICE:  QUERY PLAN:
	
	Index Scan using i_table1__field1 on table1  (cost=0.00 size=0 width=4)

The function fixing it is in gram.y:
	
	static Node *doNegate(Node *n)
	{
	    if (IsA(n, A_Const))
	    {
	        A_Const *con = (A_Const *)n;
	
	        if (con->val.type == T_Integer)
	        {
	            con->val.val.ival = -con->val.val.ival;
	            return n;
	        }
	        if (con->val.type == T_Float)
	        {
	            con->val.val.dval = -con->val.val.dval;
	            return n;
	        }
	    }
	
	    return makeA_Expr(OP, "-", NULL, n);
	}


It tries to merge the negative into the constant.  We already had
special '-' handling in the grammer, so I just call this function,
rather than doing makeA_Expr in all cases.

Committed.

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