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Each block starts life with, by default, two transaction slots. The number of simultaneous active transactions that a block can ever have is constrained by the value of MAXTRANS and by the availability of space on the block. You may not be able to achieve 255 concurrent transactions on the block if there is not sufficient space to grow this structure. We can artificially demonstrate how this works by creating a table with lots of rows packed into a single block such that the block is very full from the start; there will be very little room left on the block after we initially load our data. The presence of these rows will limit how large the transaction table can grow, due to the lack of space. I was using an 8KB block size and I tested this particular example in all versions of Oracle from 9i Release 2 through 11g Release 2 with the same results (so, if you have an 8KB blocksize, you should be able to reproduce this). We ll start by creating our packed table. I played around with different lengths of data until I arrived at this very special size: ops$tkyte%ORA11GR2> create table t 2 ( x int primary key, 3 y varchar2(4000) 4 ) 5 / Table created. ops$tkyte%ORA11GR2> insert into t (x,y) 2 select rownum, rpad('*',148,'*') 3 from dual 4 connect by level <= 46;

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Here is an example of using pattern matching with lists: let printFirst primes = match primes with | h :: t -> printfn "The first prime in the list is %d" h | [] -> printfn "No primes found in the list" > printFirst oddPrimes;; The first prime in the list is 3 val it : unit = ().

46 rows created ops$tkyte%ORA11GR2> select length(y), 2 dbms_rowidrowid_block_number(rowid) blk, 3 count(*), min(x), max(x) 4 from t 5 group by length(y), dbms_rowidrowid_block_number(rowid); LENGTH(Y) BLK COUNT(*) MIN(X) MAX(X) ---------- ---------- ---------- ---------- ---------148 4599 46 1 46 So, our table has 46 rows, all on the same block I chose 148 characters because if it was one character more, we d need two blocks to hold these same 46 records Now, we need a way to see what happens when many transactions try to lock data on this single block simultaneously For that, we ll use an AUTONOMOUS_TRANSACTION again, just so we can use a single session and not have to run lots of concurrent SQL*Plus sessions Our stored procedure will lock a row in the table by the primary key starting with a primary key value of 1 (the first record inserted).

If our procedure gets the lock on this row without having to wait (without getting blocked), it will simply increase the primary key value by 1 and, using recursion, do it all over again So, the second call will try to lock record 2, the third call record 3, and so on If the procedure is made to wait, it will raise an ORA-54 resource busy error and we ll print out locked out trying to select row <primary key value> That will indicate we ran out of transaction slots on this block before we ran out of rows to lock On the other hand, if we find no row to lock, that means we ve already locked every row on this block and we print out success (meaning, the transaction table in the block header was able to grow to accommodate all of the transactions).

> counter.Increment(2);; val it : unit = () > counter.Fetch();; val it : int = 3 > counter.Stop();; val it : unit = () Listing 13-11 shows several important aspects of message passing and processing using the mailbox-processing model: Internal messages protocols are often represented using discriminated unions. Here the type msg has cases Increment, Fetch, and Stop corresponding to the three methods accepted by the object that wraps the overall agent implementation. Pattern matching over discriminated unions gives a succinct way to process messages. A common pattern is a call to inbox.Receive() or inbox.TryReceive() followed by a match on the message contents. The PostSync on the MailboxProcessor type gives a way to post a message and wait for a reply. A temporary reply channel is created and should form part of the message. A reply channel is simply an object of type Microsoft.FSharp.Control.IChannel<'reply>, which in turn simply supports a Post method. This can be used by the MailboxProcessor to post a reply to the waiting caller. In Listing 13-11 the channel is sent to the underlying messageprocessing agent counter as part of the Fetch message. Table 13-6 summarizes the most important members available on the MailboxProcessor type.

Here is that stored procedure: ops$tkyte%ORA11GR2> create or replace procedure do_update( p_n in number ) 2 as 3 pragma autonomous_transaction; 4 l_rec t%rowtype; 5 resource_busy exception; 6 pragma exception_init( resource_busy, -54 ); 7 begin 8 select * 9 into l_rec 10 from t 11 where x = p_n 12 for update NOWAIT; 13 14 do_update( p_n+1 ); 15 commit; 16 exception 17 when resource_busy 18 then 19 dbms_outputput_line( 'locked out trying to select row ' || p_n ); 20 commit; 21 when no_data_found 22 then 23 dbms_outputput_line( 'we finished - no problems' ); 24 commit; 25 end; 26 / Procedure created..