Reg : Concept of Locks --

Question

Hi Experts,
I've got few doubts understanding the concepts of Locking.
I'm referring this article - http://docs.oracle.com/cd/E14072_01/server.112/e10592/ap_locks001.htm
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When a transaction obtains a row lock for a row, the transaction also acquires a table lock for the table in which the row resides.
The table lock prevents conflicting DDL operations that would override data changes in a current transaction.
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What is the significance of the 2 ^nd^ line? Does that mean, at that time we can't DROP or ALTER the table structure ??
Although, it is written very clear manner, i'm not able to understand it properly.
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A row share lock (RS), also called a subshare table lock (SS), indicates that the transaction holding the lock on the table has locked some rows in the table and intends to update them, as is the case in a SELECT ... FOR UPDATE statement. An SS lock is the least restrictive mode of table lock, offering the highest degree of concurrency for a table.
A row exclusive lock (RX), also called a subexclusive table lock (SX), indicates that the transaction holding the lock has made updates to rows in the table. An SX lock by itself allows other transactions to insert, update, merge into, or delete other rows in the table concurrently. Therefore, SX locks allow multiple transactions to obtain simultaneous SX and SS locks for the same table.
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- What is the need of an SX lock when it allows other transactions to manipulate data?
- Both SS and SX look like same kind of locks. Can any one please point out the difference with an example.
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A share table lock (S) held by one transaction allows other transactions to query the table but allows updates only if a share table lock is held by only a single transaction. Multiple transactions may hold a share table lock concurrently, so holding this lock is not sufficient to ensure that a transaction can modify the table.
A share row exclusive table lock (SRX), also called a share-subexclusive table lock (SSX), is more restrictive than a share table lock. Only one transaction at a time can acquire an SSX lock on a given table. An SSX lock held by a transaction allows other transactions to query or lock specific rows using SELECT ... FOR UPDATE, but not to update the table.
An exclusive table lock (X) is the most restrictive mode of table lock, allowing the transaction that holds the lock exclusive write access to the table. Only one transaction can obtain an X lock for a table.
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I'm also little confused with these locking types and not able to understand the difference practically. I know these are very important concepts and should not be neglected.
Can any one please help me understand these concepts?
Thanks In Advance,
Ranit B.
Edited by: ranit B on Nov 28, 2012 6:47 PM
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Answer 1

padders wrote:
Does that mean, at that time we can't DROP or ALTER the table structure Yes, that's pretty much what it means.
What is the need of an SX lock when it allows other transactions to manipulate data?As the text says - the lock allows concurrent update of other rows - i.e. you lock some row(s) but other rows can still be updated.Thanks Padders.
But then, what is the difference between SS and SX? Both allow other rows to be updated.
Please rectify me if I'm wrong.
I got some idea. It would be really helpful if you could please give me some pointers on the other doubts also?