What does cost mean in execution plan?

Question

What does a cost increase actually mean compared to the decrease of data that has to be processed?
More details:
I have 3 tables:
- users
- transaction types
- user transactions
I'm joining user transactions with transaction types for a particular user and aggregating some result from the transaction type table.
Originally there was no index on the user transactions table containing both the user_id (on which is filtered) and the transaction id. This lead to a TABLE ACCESS and joining a lot of data for nothing. I created an index to contain both fields so that no more TABLE ACCESS is needed. It indeed reduced the amount of data to be merged and aggregated considerably, but the COST of the query went up! This I don't understand. (the query itself did not seem to take more/less time).
What does a cost increase actually mean compared to the decrease of data that has to be processed?
Below are the two execution plans:
Execution plan with low cost and table access.
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=51 Card=1 Bytes=33)
1 0 SORT (AGGREGATE)
2 1 HASH JOIN (Cost=51 Card=283759 Bytes=9364047)
3 2 TABLE ACCESS (BY INDEX ROWID) OF 'THIRD_PARTY_TRANSACT
IONS' (Cost=2 Card=448 Bytes=8960)
4 3 INDEX (RANGE SCAN) OF 'X_TP_TRANSACTIONS_USER' (NON-
UNIQUE) (Cost=1 Card=448)
5 2 INDEX (FAST FULL SCAN) OF 'X_ISP_TRANSACTIONS_TRID_FIN
AL' (NON-UNIQUE) (Cost=4 Card=63339 Bytes=823407)
Execution plan with only index join but high cost
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=272 Card=1 Bytes=28)
1 0 SORT (AGGREGATE)
2 1 HASH JOIN (Cost=272 Card=3287 Bytes=92036)
3 2 INDEX (FAST FULL SCAN) OF 'X_TP_TRANSACTIONS_TRID_USER
ID' (NON-UNIQUE) (Cost=21 Card=3287 Bytes=49305)
4 2 INDEX (FAST FULL SCAN) OF 'X_ISP_TRANSACTIONS_TRID_FIN
AL' (NON-UNIQUE) (Cost=4 Card=63339 Bytes=823407)

Answer 1

when oracle parses and optimises a query it creates several execution plans and assigns a cost to each plan.
It then uses this cost to compare these plans.
But you can't use this cost as an indication of how "heavy" a query is, nor can you compare the costs between different queries.
It is only a value used internally by oracle.
greetings
Freek D