>Any reason why this happening.
Something is wrong.
>Is it any sqlplus environment setting or oracle server config ?
Yes, unless the source of the problem is something else.
Is it possible for you to use more words & provide nothing of value for us to consider?
>The control does not come back to the sqlplus prompt.
Never, ever? How long did you wait?
What else was happening on the system & in the OS during this time?
Are there any error messages in the alert_SID.log file?
What OS & version?
What DB version?
What is preventing you from doing cut & paste so we can actually see & KNOW exactly what you are doing?
Quantify "moderately big".
Why haven't you enabled SQL_TRACE to obtains facts WRT what actually is happening?
Other than make your post & watch nothing come back to the screen,
what have you done to solve this mystery?
You can lead some folks to knowledge, but you can not make them think.
The average person thinks he's above average!
For most folks, they don't know, what they don't know.
Good judgement comes from experience. Experience comes from bad judgement.