Set - 4

Question 86 :

Why is Oracle not using the damn index? (for DBA)

Answer :

This problem normally only arises when the query plan is being generated by the Cost Based Optimizer. The usual cause is because the CBO calculates that executing a Full Table Scan would be faster than accessing the table via the index.
Fundamental things that can be checked are:
. USER_TAB_COLUMNS.NUM_DISTINCT - This column defines the number of distinct values the column holds.
. USER_TABLES.NUM_ROWS - If NUM_DISTINCT = NUM_ROWS then using an index would be preferable to doing a FULL TABLE SCAN. As the NUM_DISTINCT decreases, the cost of using an index increase thereby is making the index less desirable.
. USER_INDEXES.CLUSTERING_FACTOR - This defines how ordered the rows are in the index. If CLUSTERING_FACTOR approaches the number of blocks in the table, the rows are ordered. If it approaches the number of rows in the table, the rows are randomly ordered. In such a case, it is unlikely that index entries in the same leaf block will point to rows in the same data blocks.
. Decrease the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT - A higher value will make the cost of a FULL TABLE SCAN cheaper.
. Remember that you MUST supply the leading column of an index, for the index to be used (unless you use a FAST FULL SCAN or SKIP SCANNING).
. There are many other factors that affect the cost, but sometimes the above can help to show why an index is not being used by the CBO. If from checking the above you still feel that the query should be using an index, try specifying an index hint. Obtain an explain plan of the query either using TKPROF with TIMED_STATISTICS, so that one can see the CPU utilization, or with AUTOTRACE to see the statistics. Compare this to the explain plan when not using an index.


Question 87 :

When should one rebuild an index? (for DBA)

Answer :

You can run the 'ANALYZE INDEX VALIDATE STRUCTURE' command on the affected indexes - each invocation of this command creates a single row in the INDEX_STATS view. This row is overwritten by the next ANALYZE INDEX command, so copy the contents of the view into a local table after each ANALYZE. The 'badness' of the index can then be judged by the ratio of 'DEL_LF_ROWS' to 'LF_ROWS'.


Question 88 :

What are the unrestricted procedures used to change the popup screen position during run time ?

Answer :

Anchor-view
Resize -View
Move-View.


Question 89 :

What is an Alert ?

Answer :

An alert is window that appears in the middle of the screen overlaying a portion of the current display.


Question 90 :

Deleting a page removes information about all the fields in that page ?
a. True. b. False

Answer :

a. True.