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USERNAME SID RBS SLOT SEQ LMODE REQUEST --------- ---------- ---------- ---------- ---------- ---------- ---------OPS$TKYTE 7 2 26 8297 0 6 OPS$TKYTE 703 2 26 8297 6 0 OPS$TKYTE 7 7 13 8215 6 0 ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> select XIDUSN, XIDSLOT, XIDSQN 2 from v$transaction; XIDUSN XIDSLOT XIDSQN ---------- ---------- ---------2 26 8297 7 13 8215 What we see here is that a new transaction has begun, with a transaction ID of (7,13,8215). Our new session, SID=7, has two rows in V$LOCK this time. One row represents the locks that it owns (where LMODE=6). It also has a row that shows a REQUEST with a value of 6. This is a request for an exclusive lock. The interesting thing to note here is that the RBS/SLOT/SEQ values of this request row are the transaction ID of the holder of the lock. The transaction with SID=703 is blocking the transaction with SID=7. We can see this more explicitly simply by doing a self-join of V$LOCK: ops$tkyte%ORA11GR2> select 2 (select username from v$session where sid=a.sid) blocker, 3 a.sid, 4 ' is blocking ', 5 (select username from v$session where sid=b.sid) blockee, 6 b.sid 7 from v$lock a, v$lock b 8 where a.block = 1 9 and b.request > 0 10 and a.id1 = b.id1 11 and a.id2 = b.id2; BLOCKER SID 'ISBLOCKING' BLOCKEE SID --------- ---------- ------------- --------- ---------OPS$TKYTE 703 is blocking OPS$TKYTE 7 Now, if we commit our original transaction, SID=703, and rerun our lock query, we find that the request row has gone: ops$tkyte%ORA11GR2> select username, 2 v$lock.sid, 3 trunc(id1/power(2,16)) rbs, 4 bitand(id1,to_number('ffff','xxxx'))+0 slot, 5 id2 seq, 6 lmode, 7 request 8 from v$lock, v$session 9 where v$lock.type = 'TX' 10 and v$lock.sid = v$session.sid 11 and v$session.username = USER;

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F# lists are represented in memory as linked lists, and each F# list value is a cons cell containing a value plus a pointer to the next chain in the list, or else it is a special nil object When you create a new list using the :: operator, then the tail of the new list will point to the old list, which ensures that the inner memory associated with lists is often reused as part of multiple list values You can decompose lists from the head downward by using pattern matching You saw some simple examples of pattern matching on tuples in 2, and we ll look at pattern matching in more detail in Getting Started with Pattern Matching later in this chapter.

USERNAME SID RBS SLOT SEQ LMODE REQUEST --------- ---------- ---------- ---------- ---------- ---------- ---------OPS$TKYTE 7 7 13 8215 6 0 ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> select XIDUSN, XIDSLOT, XIDSQN 2 from v$transaction; XIDUSN XIDSLOT XIDSQN ---------- ---------- ---------7 13 8215 The request row disappeared the instant the other session gave up its lock. That request row was the queuing mechanism. The database is able to wake up the blocked sessions the instant the transaction is completed. There are prettier displays with various GUI tools, but in a pinch, having knowledge of the tables you need to look at is very useful. However, before we can say that we have a good understanding of how the row locking in Oracle works, we must look at one last topic: how the locking and transaction information is managed with the data itself. It is part of the block overhead. In 10 Database Tables , we ll get into the details of the block format, but suffice it to say that at the top of a database block is some leading overhead space in which to store a transaction table for that block. This transaction table contains an entry for each real transaction that has locked some data in that block. The size of this structure is controlled by two physical attribute parameters on the CREATE statement for an object: INITRANS: The initial, preallocated size of this structure. This defaults to 2 for indexes and tables. MAXTRANS: The maximum size to which this structure may grow. It defaults to 255 and has a minimum of 2 practically. In Oracle 10g and above, this setting has been deprecated, so it no longer applies. MAXTRANS is 255 regardless in that release and later.