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Inequality and NULL

A response recent forums question about handling an inequality condition also reminded me, once again, about NULL handling in Oracle.

You can have :

WHERE column = 100 

in a query.

In another query, you could have

WHERE column != 100 

However, if the column does contain a NULL in any one (or more rows), the union of the two queries is not the entire table. That is, rows with NULLs are excluded.
You have to specifically handle NULLs with either

SQL Injection

Here's a very simple example of SQL Injection by Mike Smithers.


New Presentation : On Nested Loop and Hash Join

I have uploaded a new presentation on Nested Loop and Hash Joins. This is based on the "Nested Loops and Consistent Gets" case study.


Getting the right statistics

Continuing with the post titled "Nested Loops and Consistent Gets" earlier today, what is the Execution Plan with statistics on more columns ?

SQL> exec dbms_stats.gather_table_stats('','TRANSACTIONS',estimate_percent=>100,granularity=>'ALL',-
> method_opt=>'FOR COLUMNS COUNTRY_CD SIZE 250, PRODUCT_CD SIZE 250, TXN_ID SIZE 250 ',cascade=>TRUE);

PL/SQL procedure successfully completed.


Nested Loop and Consistent Gets

DBAs with a few years of experience and/or having read some material on performance tuning know that eliminating physical reads and achieving a high buffer cache hit ratio isn't necessarily a tuning goal.
Some DBAs have accepted the idea that, rather than physical reads, they should concentrate on logical reads -- consistent gets in case of queries.
But ...... there really is no "absolutely correct optimization target". Everything is relative.

Interpreting an AWR report when the ArchiveLog Dest or FRA was full

It can be very interesting / challenging / confusing / misleading / wrong to attempt to interpret an AWR report for the duration when ArchiveLogs could not be generated. Such a case is when the ArchiveLog Destination directory or FRA (whichever is in use) went to 100% full usage (or hit the db_recovery_file_dest_size and Oracle cannot auto-delete any files in the FRA, all files present being necessary to meet the RETENTION Policy).

Deleting SQL Plan Baselines

Following the example of capturing SQL Plan Baselines, here's a deletion :

SQL> select sql_handle, sql_text from dba_sql_plan_baselines

------------------------------ --------------------------------------------------------------------------------
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