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F# lists are a common data structure used in functional programming. You saw some examples of concrete lists when using the results of the String.split function in 2. Table 3-8 shows the primitive constructs for building lists.

Search through the list to see if anyone else has locked this row. Create a new entry in the list to establish the fact that you have locked the row. Unlock the list.

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Now that you have the row locked, you can modify it. Later, as you commit your changes, you must continue the procedure as follows: 1. 2. 3. 4. Get in line again. Lock the list of locks. Search through the list and release all of your locks. Unlock the list.

The empty list Cons an element with a list A list value A range of integers A generated list (see end of chapter) Concatenates two lists

As you can see, the more locks acquired, the more time spent on this operation, both before and after modifying the data. Oracle does not do it that way. Oracle s process looks like this: 1. 2. 3. Find the address of the row you want to lock. Go to the row. Lock the row right there, right then at the location of the row, not in a big list somewhere (waiting for the transaction that has it locked to end if it is already locked, unless you are using the NOWAIT option).

[] 1 :: [2; 3] [1; 2; 3] [1 .. 99] [ for x in 1..99 -> x * x ] [1; 2] @ [3]

That s it Since the lock is stored as an attribute of the data, Oracle does not need a traditional lock manager The transaction will simply go to the data and lock it (if it is not locked already) The interesting thing is that the data may appear locked when you get to it, even if it s not When you lock rows of data in Oracle, the row points to a copy of the transaction ID that is stored with the block containing the data, and when the lock is released that transaction ID is left behind This transaction ID is unique to your transaction and represents the undo segment number, slot, and sequence number You leave that on the block that contains your row to tell other sessions that you own this data (not all of the data on the block just the one row you are modifying).

When another session comes along, it sees the transaction ID and, using the fact that it represents a transaction, it can quickly see if the transaction holding the lock is still active If the lock is not active, the session is allowed access to the data If the lock is still active, that session will ask to be notified as soon as the lock is released Hence, you have a queuing mechanism: the session requesting the lock will be queued up waiting for that transaction to complete, and then it will get the data Here is a small example showing how this happens, using three V $ tables: V$TRANSACTION, which contains an entry for every active transaction V$SESSION, which shows the sessions logged in V$LOCK, which contains an entry for all enqueue locks being held as well as for sessions that are waiting on locks.

Here are some basic list values: let oddPrimes = [3; 5; 7; 11] let morePrimes = [13; 17] let primes = 2 :: (oddPrimes @ morePrimes) The value and type of primes are as follows:

You will not see a row in this view for each row locked by a session As stated earlier, that master list of locks at the row level doesn t exist If a session has one row in the EMP table locked, there will be one row in this view for that session indicating that fact If a session has millions of rows in the EMP table locked, there will still be just one row in this view This view shows what enqueue locks individual sessions have..

First, let s get a copy of the EMP and DEPT tables. If you already have them in your schema, replace them with the following definitions:

val primes : int list = [2; 3; 5; 7; 11; 13; 17]