SQL Tuning

I am facing a problem.

One particular simple join (on UK with 7,000 rec) is increasing the cost drastically (~ 165759) without the join SQL cost is 1387.

The only differene I find is with the with it has 3 HASH JOINS and without only one.

Can anybody point me out what's happening?. What is HASH JOIN.

Thanks in advance

Amitabh

Cost with Join:
  0 SELECT STATEMENT Optimizer=CHOOSE (Cost=165759 Card=58 Bytes=33408)

   1 0 FILTER

   2    1     SORT (GROUP BY) (Cost=165759 Card=58 Bytes=33408)
   3    2       FILTER
   4    3         HASH JOIN (Cost=1018 Card=178563 Bytes=102852288)
   5    4           TABLE ACCESS (FULL) OF 'COCOPHDR' (Cost=50
Card=9276 Bytes=296832)
   6    4           HASH JOIN (OUTER) (Cost=126 Card=1925
Bytes=1047200)
   7    6             NESTED LOOPS (OUTER) (Cost=93 Card=139
Bytes=71168)
   8    7               HASH JOIN (Cost=83 Card=10 Bytes=4800)
   9    8                 TABLE ACCESS (FULL) OF 'PE_ORIG_DEST_MSTR'
(Cost=6 Card=70 Bytes=2240)
  10    8                 NESTED LOOPS (Cost=76 Card=14 Bytes=6272)
  11   10                   NESTED LOOPS (OUTER) (Cost=75 Card=1

Cost without join:

   0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1387 Card=1 Bytes=537)

   1 0 FILTER

   2    1     SORT (GROUP BY) (Cost=1387 Card=1 Bytes=537)
   3    2       FILTER
   4    3         HASH JOIN (Cost=128 Card=2687 Bytes=1442919)
   5    4           TABLE ACCESS (FULL) OF 'PE_ORIG_DEST_MSTR' (Cost=6
Card=1385 Bytes=44320)
   6    4           NESTED LOOPS (OUTER) (Cost=91 Card=194
Bytes=97970)
   7    6             NESTED LOOPS (OUTER) (Cost=77 Card=14
Bytes=6622)
   8    7               NESTED LOOPS (OUTER) (Cost=76 Card=1
Bytes=441)
   9    8                 NESTED LOOPS (OUTER) (Cost=75 Card=1
Bytes=407)
  10    9                   NESTED LOOPS (OUTER) (Cost=74 Card=1
Bytes=373)
  11   10                     NESTED LOOPS (OUTER) (Cost=73 Card=1