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  Oracle Tips by Burleson


So, you go to the library to find a book to read.  It is summer, and you have time on your hands and are not sure what you want.  You probably browse the shelves looking for something that might interest you.  When you’re not sure what you want, this is a pretty easy access method, browsing the shelves.  But a database never browses for a row. It is always looking for a particular row or rows. 

So, let’s go back to the library and look for a particular book.  In this instance, you would not go and browse the shelves; you would head for the card catalog.  It could take you hours to find a book by searching the shelves, while it will take only minutes to look up the book and go directly to the shelf where the book is located.  This is because all of the books in the library are numbered using the Dewey Decimal system.  Sometimes, you are not sure which book you want but know the general subject.  You can look up the subject in the card catalog and then go to the shelves and look at a range of books. 

Oracle uses basically the same system, except that the card catalog is an index and the book number becomes the rowid.  When a query asks for a specific row of data, an index allows the database to look up the rowid of that specific row and directly retrieve it.  Otherwise, the database must start at the beginning of the table and check each row to find the data it wants.

select *
from author
where author_key = 'A104';

In the example above, I am looking for a row in the AUTHOR table where the author_key is A104.  If I have an index on the author key, the database will use that index to quickly find the correct row.

----------- ---------------------------------------- -----AUTHOR_PHONEAUTHOR_STREET           AUTHOR_CITY         AU
------------ ---------------------------------------- ----AUTHO AUTHOR_CONTRACT_NBR
----- -------------------
A104        jeckle                   pierre
543-333-9241 3671 old fort st      north hollywood      CA
91607                6602

Execution Plan
----------------------------------------------------------   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1
Card=1 Bytes=73)
(TABLE) (Cost=1
          Card=1 Bytes=73)
          t=0 Card=1)

In the example above, I pulled the execution plan from the database.  Notice that the database used the author_pk index (which is on the author key) to find the rowed, and then performed a table access by rowid to retrieve the row (read from inside to outside).  It basically looked up the book title (author_key) in the card catalog (index) and went directly to the shelve (table) and retrieved the book (row). 

So, an index is a pairing of a column (or columns) value and the rowid for that row.  In our example with the PUBS database, there is not much difference between using an index and scanning the entire table.  That is because we only have ten or so rows of data in our table.  If we had hundreds of thousands of rows, the index would show a significant speed improvement over scanning the entire table.  If there were only ten books in the library, you would also skip the card catalog.  An index can also provide a range if values for the database to retrieve from the table.   Let’s look at an index and how the database creates it.

SQL> create index sales_book
  2  on sales (book_key); 

Index created.

In the example above, we created an index called sales_book on the book key of the SALES table.  We chose this index because many of our queries use the book_key in the WHERE clause filters.  Since the query is filtering by book key, it can use the index and collect the rowids of rows that pass the filters. 

Indexes also speed UPDATEs and DELETEs.  Remember, to DELETE or UPDATE  a row, the database must first find the row.  The index helps the database quickly find the row.

The above book excerpt is from:

Easy Oracle SQL

Get Started Fast writing SQL Reports with SQL*Plus

ISBN 0-9727513-7-8

Col. John Garmany 



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