explain analyze cost

[Ravi Krishna]

I am running explain analyze cost on a SQL which reads from two large
tables (122mil and 37 mil).  The query is an UPDATE SQL where we use
derives table in the from clause and then join it back to the table
being updated.
The explain analyze cost itself is taking forever to run. It is running
for the last 1 hr. Does that actually run the SQL to find out the
impact of I/O (as indicated in COSTS). If not, what can cause it to run
this slow.__