Question

How much time should be taken by query in oracle if cost is 77??

Answer 1

See the Oracle Performance Tuning Guide:

The cost is an estimated value
proportional to the expected resource
use needed to execute the statement
with a particular plan. The optimizer
calculates the cost of access paths
and join orders based on the estimated
computer resources, which includes
I/O, CPU, and memory.

Serial plans with higher costs take
more time to execute than those with
smaller costs. When using a parallel
plan, however, resource use is not
directly related to elapsed time.

and

The cost represents units of work or
resource used. The query optimizer
uses disk I/O, CPU usage, and memory
usage as units of work. So, the cost
used by the query optimizer represents
an estimate of the number of disk I/Os
and the amount of CPU and memory used
in performing an operation. The
operation can be scanning a table,
accessing rows from a table by using
an index, joining two tables together,
or sorting a row set. The cost of a
query plan is the number of work units
that are expected to be incurred when
the query is executed and its result
produced.