oracle internal

当oracle遇到问题时, 当表面的现象和现有的log无法为我们诊断问题提供足够的信息时, 希望可能通过打开oracle 的debug开关,生成更详细的trace 文件提供更多的信息, 这里整理了一些trace的命令.

首先查看表的统计信息，分区表的统计信息分global stats(DBA_TABLE)和partition stats(DBA_TAB_PARTITION),发现该分区表是global stats 有，partition stats全为空， 查看该SQL发现有两个子游标有不同的执行计划

By default DBUA removes all the hidden parameters during upgrade.If you want to keep the hidden parametersinvoke DBUA with ‘keepHiddenParams’ option.e.g./dbua -keepHiddenParams

If you need to recovery the dead transaction record of more than 100(_cleanup_rollback_entries parameter value), PMON only responsible for cleaning up dead session, which is responsible for the SMON recovery dead transaction.

scn(system change number) 结构主要维护 oracle数据库内部的数据一致性，SCN 有两部分组成: Base and wrap，wrap 是16bit的数字，base是32bit的数字,这样其实就可以算scn的有效范围，它的格式(redo dump trace)是wrap.base，当base超过了2的32次方,然后wrap 就会加1，其实用sql 很好验证。

It is widely known that the Oracle server uses kslgetl – Kernel Service Lock Management Get Latch function to acquire the latch,oradebug call kslgetl/kslfre can be used to acquire the latch manually. This is very useful to simulate latch related hangs and contention.

ITL(Interested Transaction List ),The Oracle database engine locks the data modified by a transaction at the row level. To implement this feature every data block contains a list of all transactions that are modifying it. This list is commonly called interested transaction list (ITL).

Both “db file sequential read” and “db file scattered read” events signify time waited for I/O read requests to complete.

when a long, running transaction has been rolled back, How Much Longer completion time of the rollback process? The questions are justified, because the transaction holds the locks and normal processing often suffers as the rollback progresses.

Session cursor caching does not mean that server process caches the whole SQL area in the PGA.It just means that Oracle does not release the cursor object when the same statement was executed more than 2 times in the same process…

When job execution fails, the SNP processes attempts to rerun the job one minute later. If this run fails, another attempt is made in two minutes and another in four minutes. The job queue doubles the retry interval until it exceeds the normal execution interval..

有时总是忽略在列的顺序带来的性能开销，还有列的null值字段的位置,想理解这个问题要首先程序列数据是如何在block上存储的，并且是如何定位的？

为了保证分布式查询的一致性，通过dblink查询时会同步SCN，有时会撞见ORA-600错误。
alert日志中伴随着warning Rejected the attempt to advance SCN over limit

转储了oracle block的值，如何得到它真正表中的值，也算 是dump(val,16)的逆运算，只含char,number,date