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to this statement:
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SELECT * FROM Table1 WHERE indexed_column > 0
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This is a way of forcing the DBMS to look up via the index Alas, it works only with a few DBMSs In general, then, don't add redundant conditions to WHERE clauses
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Ensure You Use the Right DBMS
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There are several ways to ensure that a specific DBMS (and no other) executes an expression Here are three examples, all of which use nonstandard SQL extensions:
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Example 1: WHERE :variable = 'Oracle' AND /* Oracle-specific code here */ Example 2: SELECT /* ! HIGH_PRIORITY */ /* all DBMSs except MySQL ignore this */ Example 3: WHERE <escape-sequence> AND /* ODBC code */
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While we're on the subject, Oracle allows you to add a comment that indicates what index you want to use It looks like this:
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SELECT /*+ INDEX(Widgets Widget_index) */ column1, column2, column3 FROM Widgets WHERE column1 <> 7; GAIN: only 1/8 because it's Oracle-specific
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Oracle-specific optimizations are bad ideas if they tie you to Oracle In this case, the hint is in a comment, so other DBMSs will ignore it That's good it's more portable than putting hints outside comments as Microsoft and Sybase do So it's okay until other DBMSs start to put executable data inside comments too Some are already starting to do so So right now hints are okay, but eventually they will lead to conflict and chaos
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Constant Folding
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"Go north one mile, west one mile, west one more mile, then south one mile, and you will be at your starting point" The South Pole Riddle Anyone who has used C will know that the expression x=1+1-1-1 is folded to x=0 at compile time So it may surprise you that many SQL DBMSs do not fold these five obvious-looking transform candidates:
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WHERE column1 + 0 WHERE 5 + 00 WHERE column1 IN (1, 3, 3) CAST(1 AS INTEGER) WHERE 'a' || 'b'
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If you find expressions like these in old code though, our tip is Leave them alone They are there for historical reasons, such as forcing the DBMS to ignore indexes, changing the result data type, allowing for the difference between SMALLINT and INTEGER, or evading a limit on line size Sorry but the obvious-looking cases are precisely the cases where you should stop and wonder whether the original programmer had some reason for the weird syntax choice We read a Java optimization article once that sums it up nicely: "Rule #1: Understand the code" Nevertheless, we do recommend that you transform this search condition:
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WHERE a - 3 = 5
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WHERE a = 8 GAIN: 6/8
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/* a - 3 = 5 */
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Although it's useless in simple cases, constant folding can lead to constant propagation and is therefore A Good Thing
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Case-Insensitive Searches
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Microsoft's Access Jet considers the strings 'SMITH' and 'Smith' to be equal, so Access is called a case-insensitive DBMS Oracle, on the other hand, is usually case sensitive (the engine would say 'SMITH' and 'Smith' are unequal strings) Sybase allows you to decide about case sensitivity when you install, and a true SQL Standard DBMS will allow you to switch case sensitivity at runtime We've seen many programmers try to ensure case insensitivity by using the fold function UPPER, as in:
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WHERE UPPER(column1) = 'SMITH'
That can be a mistake if you're dealing with strings that contain anything other than strictly Latin letters With some DBMSs, when you translate certain French or German strings to uppercase, you lose information For example, the function:
UPPER('r sum ')
returns RESUME that is, the accent marks are lost, changing the meaning of the word from "curriculum vitae" to "begin again" Because information isn't lost going the other way, it's better to use the LOWER function, like this:
WHERE LOWER(column1) = 'r sum '
An even better way is to eliminate the fold function entirely if that's possible, because we appeal to authority here both the Microsoft and Oracle manuals say: "Avoid functions on columns" We're sure they mean "avoid functions on columns when there's another way to get the result needed" for example, to ensure case insensitivity, the best method is to use a case-insensitive collation rather than a fold function A slightly faster search assumes that the data is clean and asks for the only reasonable combinations, like this:
WHERE column1 = 'SMITH' OR column1 = 'Smith' GAIN: 8/8
which is still slow Our tip here is Take advantage of dead code elimination so that the 'Smith' search happens only when the DBMS is case sensitive Here's how:
WHERE column1 = 'SMITH' OR ('SMITH' <> 'Smith' AND column1 = 'Smith') GAIN: 3/8
Sargability "CHUFFED adj [1] Pleased [2] Displeased" The Concise Oxford Dictionary The ideal SQL search condition has the general form: