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CHAPTER 6 LOCKING AND LATCHING
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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.
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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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CHAPTER 6 LOCKING AND LATCHING
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46 rows created. ops$tkyte%ORA11GR2> select length(y), 2 dbms_rowid.rowid_block_number(rowid) blk, 3 count(*), min(x), max(x) 4 from t 5 group by length(y), dbms_rowid.rowid_block_number(rowid); LENGTH(Y) BLK COUNT(*) MIN(X) MAX(X) ---------- ---------- ---------- ---------- ---------148 4599 46 1 46 So, our table has 46 rows, all on the same block. I chose 148 characters because if it was one character more, we d need two blocks to hold these same 46 records. Now, we need a way to see what happens when many transactions try to lock data on this single block simultaneously. For that, we ll use an AUTONOMOUS_TRANSACTION again, just so we can use a single session and not have to run lots of concurrent SQL*Plus sessions. Our stored procedure will lock a row in the table by the primary key starting with a primary key value of 1 (the first record inserted). If our procedure gets the lock on this row without having to wait (without getting blocked), it will simply increase the primary key value by 1 and, using recursion, do it all over again. So, the second call will try to lock record 2, the third call record 3, and so on. If the procedure is made to wait, it will raise an ORA-54 resource busy error and we ll print out locked out trying to select row <primary key value> . That will indicate we ran out of transaction slots on this block before we ran out of rows to lock. On the other hand, if we find no row to lock, that means we ve already locked every row on this block and we print out success (meaning, the transaction table in the block header was able to grow to accommodate all of the transactions). Here is that stored procedure: ops$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_output.put_line( 'locked out trying to select row ' || p_n ); 20 commit; 21 when no_data_found 22 then 23 dbms_output.put_line( 'we finished - no problems' ); 24 commit; 25 end; 26 / Procedure created.
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