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Selective criteria in your WHERE clauses can dramatically decrease the amount of data Oracle has to consider during a query You can follow some simple principles to ensure that the structure of your SQL statements is not inherently inefficient Your join methods may be fine, but overlooking some of these principles could doom your statement from a performance point of view Careful specification of WHERE conditions can have a significant bearing on whether the optimizer will choose existing indexes The principle of selectivity the number of rows returned by a query as a percentage of the total number of rows in a table is the key idea here A low percentage means high selectivity and a high percentage means the reverse Because more selective WHERE clauses mean fewer I/Os, the CBO tends to prefer to choose those kinds of WHERE clauses over others in the same query.

The following example makes this clear: SQL> SELECT * FROM national_employees WHERE ss_no = 515086789 AND city='DALLAS'; Two WHERE clauses are in this example, but you can see that the first WHERE clause that uses ss_no requires fewer I/Os The column ss_no is the primary key and is highly selective only one row with that ss_no is in the entire table The optimizer determines the selectivity of each of the two columns in the query by looking at the index statistics, which tell it how many rows in the table contain each of the two column values in the query If neither of the columns has an index, Oracle will use a full table scan to retrieve the answer to the query If both of them have indexes, it will use the more selective (and hence more efficient) index on the ss_no column.

Delete Shared File()

If you think that the optimizer should have used an index instead of doing a full table scan, then perform the following steps: 1 Views in a query sometimes prevent the use of indexes Check to make sure that the execution plan shows that the correct indexes are being used..

2. If you think heavy data skew is in the table, use histograms to provide Oracle with a more accurate representation of the data distribution in the table. The CBO assumes a uniform distribution of column data. The CBO may forego the use of an index even when a column value is selective, because the column itself is unselective in nature. Histograms help by providing the CBO with an accurate picture of the column data distribution. I discuss histograms later in this chapter, in the section Using Histograms. 3. If Oracle is still refusing to use the index, force it to do so by using an index hint, as explained in the section Using Hints to Influence the Execution Plan later in this chapter.

Figure 8-16. Using a shared file between processes on the same machine The sender writes the notification payload to a temporary file in the shared folder. Once the file has been written, its name is changed into the shared file s designated name. The file system executes the Rename command as an uninterruptible command, so the receiver doesn t see the shared file appear until the temp file has been completely written, closed, renamed, and is ready to be read. The renaming stratagem spares the sender and receiver from having to deal with file locks. A significant problem with this simplistic approach is that the sender can t send further notifications until the subscriber has read the previous one. There are two typical solutions. The simplest is to use multiple files, named according to a numbering scheme. The alternative is to use a single file, but with new rules, to allow the file to contain multiple notifications. Using multiple files, the sender uses the same approach shown in the previous sequence diagram, but creates a new shared file for each notification sent, using a predefined naming sequence. It might create files named SharedFile000, SharedFile001, etc. The receiver would delete each file after reading it. Listing 8-16 and Listing 8-17 show implementations in C# and VB .NET. Listing 8-16. Using Numbered Files to Fire Events in C# public class Sender { string sharedDirectory; int fileNumber = 0; public Sender(string theSharedDirectory) { sharedDirectory = theSharedDirectory; if (!System.IO.Directory.Exists(theSharedDirectory)) System.IO.Directory.CreateDirectory(theSharedDirectory); } public void FireEvent(string thePayload) { string tempFileName = sharedDirectory + @"\temp"; // create the temp file System.IO.StreamWriter writer = new System.IO.StreamWriter(tempFileName); writer.WriteLine(thePayload); writer.Close();

It isn t always obvious why Oracle doesn t use an index. For example, Oracle may not use an index because the indexed columns are part of an IN list, and the consequent transformation prevents the use of an index.