Encouraging Index Use in Java

Encode Data Matrix 2d barcode in Java Encouraging Index Use
Encouraging Index Use
Data Matrix ECC200 Printer In Java
Using Barcode maker for Java Control to generate, create Data Matrix image in Java applications.
People often associate sorting with indexing Partly that's because at one time sorting was indexing dBASE II seems to have performed sorts by creating indexes Nowadays, sorting is a separate activity, and only transient associations exist between the two (For example, the DBMS may perform a sort while processing a CREATE INDEX statement) If the DBMS uses an index on column1, the natural result is that the rows will come out in order by column1 So it's tempting to replace this SQL statement:
Barcode Maker In Java
Using Barcode printer for Java Control to generate, create barcode image in Java applications.
SELECT * FROM Table1 ORDER BY column1
Scan Barcode In Java
Using Barcode decoder for Java Control to read, scan read, scan image in Java applications.
with this one:
Data Matrix ECC200 Drawer In Visual C#
Using Barcode maker for Visual Studio .NET Control to generate, create ECC200 image in Visual Studio .NET applications.
SELECT * FROM Table1 WHERE column1 >= '' ORDER BY column1 GAIN: 5/8
Paint Data Matrix 2d Barcode In Visual Studio .NET
Using Barcode creator for ASP.NET Control to generate, create DataMatrix image in ASP.NET applications.
The idea here is that ORDER BY works a bit faster if some preordering occurs before the sort And, in fact, a gain occurs when the WHERE clause is used There are, however, three things to note about this trick:
Paint ECC200 In Visual Studio .NET
Using Barcode creation for VS .NET Control to generate, create Data Matrix ECC200 image in Visual Studio .NET applications.
You still can't omit the ORDER BY clause because there is no guarantee that the DBMS will use the index The WHERE clause eliminates NULL values not necessarily the result you want In Oracle, there is no such thing as a truly descending index (the keyword DESC is allowed but ignored), so you can use this idea only for ascending sorts
ECC200 Drawer In VB.NET
Using Barcode generation for .NET Control to generate, create Data Matrix 2d barcode image in .NET framework applications.
There is a variation on the trick, though Suppose you have a compound index on (column1, column2) You want to retrieve by column2, but sort by column1 Here's how:
Bar Code Creation In Java
Using Barcode encoder for Java Control to generate, create bar code image in Java applications.
SELECT column1, column2 FROM Table1 WHERE column1 > 0 AND column2 = <result you want> ORDER BY column1
Barcode Maker In Java
Using Barcode encoder for Java Control to generate, create barcode image in Java applications.
As we saw in 2, "Simple Searches," this is called using a redundant expression It works particularly well if the DBMS chooses to use the index on (column1, column2) as a covering index (discussed in 9, "Indexes") A second variation on the trick works if your DBMS supports clustered indexes In that case, you can sort the columns of a table's cluster key in a way that supports your favorite ORDER BY option When a clustered index exists, the rows are in order by the cluster key this is guaranteed This is not to say that you should try to force the DBMS to use an index merely because it helps with ORDER BY The DBMS can choose another path Exceptionally, you might want to override the DBMS's path choice if your main interest is to get the first rows onto the user screen as quickly as possible Microsoft specially allows for that possibility with its FIRSTFASTROW hint This is particularly important if you want to limit the number of rows A question can arise whether you should change your ORDER BY clause to suit existing indexes or make new indexes to support your ORDER BY plans In both cases, we would have to say "No" If you did either one, you'd be crossing the boundary between "taking advantage of a side effect" and "depending on a side effect" Anyway, recall from the discussion of secondary sorts earlier in this chapter, that a column's index keys may not be in the same order that you will need when you sort the column DBMSs can use compound indexes For example:
UPC-A Creator In Java
Using Barcode printer for Java Control to generate, create GS1 - 12 image in Java applications.
SELECT column1, column2 FROM Table1 ORDER BY column1
Code 128 Code Set B Maker In Java
Using Barcode creation for Java Control to generate, create Code 128 Code Set B image in Java applications.
will be faster if a compound index (column1, column2) is on Table1 (GAIN: 5/8 with compound index) DBMSs can misuse noncompound indexes For example:
Code 39 Extended Creation In Java
Using Barcode creator for Java Control to generate, create Code 3/9 image in Java applications.
SELECT column1, column2 FROM Table1 ORDER BY column1, column2
Creating British Royal Mail 4-State Customer Code In Java
Using Barcode drawer for Java Control to generate, create RoyalMail4SCC image in Java applications.
will be slower if a noncompound index (column1) is on Table1 (GAIN: 3/8) To improve the sort speed, remove the index entirely or replace it with a compound index (GAIN: 3/8)
Print Code-128 In VS .NET
Using Barcode drawer for ASP.NET Control to generate, create Code 128A image in ASP.NET applications.
Preordering
Drawing Bar Code In Visual C#
Using Barcode generation for .NET Control to generate, create barcode image in .NET applications.
We have touched a few times on the point that ORDER BY goes more quickly if the incoming data is presorted but not a lot more quickly, so this section involves ideas that pay off in only a minority of cases The obvious ways to preorder by column1 are either (a) declare that column1 is the clustered index key, or (b) export in sorted order and reimport The primary advantage here is that fewer disk seeks should occur when the fetch direction is always forward A further gain could occur if you split the sorted table into two, since some DBMSs can perform sorts in parallel if they're on two separate tables The less obvious way to preorder is to add a "rough key" column to the table The procedure to form rough keys is analogous to the one we showed you for sort keys in Listing 3-1 The difference is that the rough key must be a single integer that contains only some primary-sort information for the first few characters of a column If you decide to use this idea, keep this tip in mind: A 32-bit integer can store up to five uppercase Latin characters, given six bits per character Is the second of these two statements really faster than the first
Code-39 Encoder In Visual Basic .NET
Using Barcode generation for .NET Control to generate, create Code 3 of 9 image in VS .NET applications.
SELECT * FROM Table1 ORDER BY real_column SELECT * FROM Table1 ORDER BY rough_key, real_column GAIN: 5/8
Make UPC-A Supplement 2 In .NET Framework
Using Barcode drawer for ASP.NET Control to generate, create Universal Product Code version A image in ASP.NET applications.
The answer is definitely "Yes" if duplication of column values is rare Is the saved time worth the trouble of making the rough key column The answer is "Maybe yes" if absolutely every SELECT on the table has an ORDER BY real_column clause tacked on to it
Data Matrix Generator In VB.NET
Using Barcode creator for .NET Control to generate, create ECC200 image in .NET applications.
Bar Code Generation In Visual Studio .NET
Using Barcode creation for ASP.NET Control to generate, create bar code image in ASP.NET applications.