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WHERE title LIKE 'The %' OR title LIKE 'A %' WHERE name <> 'Smith' WHERE number = 5
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Although the slowest search conditions are those that contain joins and subqueries, this chapter deals only with single-table searches (We'll talk about joins and subqueries later) Also, although search conditions can appear in HAVING, IF, or ON clauses, we'll talk only about search conditions in WHERE clauses So the chapter title "Simple Searches" is an understatement Don't worry the complex queries will come later
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In this part of the chapter, we'll look at some general ideas you should keep in mind when writing simple search conditions
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The best search conditions are those that work on few rows with easy-to-do comparisons Table 2-1 and Table 2-2 show typical lists (derived from vendors' manuals) of types of search conditions, in order from best to worst Each search condition component has a "point count" the better the component, the higher the score You can see from the allotted points shown in Tables 2-1 and 2-2 that the best search condition is something like:
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WHERE smallint_column = 12345
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Operator = > >= < <= LIKE <> Operand Table 2-1 Search Condition Point Counts for Operators Points 10 5 5 5 5 3 0 Table 2-2 Search Condition Point Counts for Operands Points
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Literal alone Column alone Parameter alone Multioperand expression Exact numeric data type Other numeric data type Temporal data type Character data type NULL This example gets a total of 27 points, calculated as follows:
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Five points for the column (smallint_column) alone on the left Two points for the exact numeric (smallint_column) operand data type Ten points for the equals operator Ten points for the literal (12345) alone on the right
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Here's another example:
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WHERE char_column >= varchar_column || 'x'
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The point count for this type of search condition is much lower only 13:
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Five points for the column (char_column) alone on the left Zero points for the CHAR (char_column) operand data type Five points for the greater than or equal to operator Three points for the multioperand expression (varchar_column || 'x') on the right Zero points for the VARCHAR (varchar_column) operand data type
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The precise point count for a search condition varies from vendor to vendor, so it's pointless to memorize anything other than the order and the concept for this optimization technique So just remember: The condition that takes the least time usually because it involves fewer rows or easier comparisons gets the most points
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Armed with this concept, you can decide whether to change the order of expressions, or to substitute one expression for another that does the same work Even though a modern cost-based DBMS optimizer has many more rules that require information outside the SQL statement itself, all DBMSs still fall back on the point count when no other information is available The possibility is always there that you will use an item of information that the optimizer doesn't (For more information about costbased optimizers, see 17 "Cost-Based Optimizers") Another way you can optimize a search condition is by putting multiple expressions in the correct order The expressions in this WHERE clause are already in optimal order:
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SELECT * FROM Table1 WHERE column1 = 5 AND column2 = 773 AND column3 = 'Smith' AND column4 < 117 AND column4 > column5 GAIN: 0/8
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The note at the bottom of this example says there is a GAIN: 0/8 That's an important number, and we're going to say "GAIN: x/8" in many of our examples, so let's clarify As explained in 1, the gain shows how many of the Big Eight run faster when the search condition is optimized Mileage varies with different data and different machines, of course We're only reporting what our tests showed So "GAIN: 0/8" means "you'd be wasting your time if you rearranged this particular WHERE clause into optimum order because the DBMS does this for you" All DBMS makers know the basics of point counting, so the rearrangement is automatic This means that, in ordinary cases, you will gain nothing by doing your own syntax-based optimization However, there are many exceptions to the rule For the rest of this chapter, we'll look at cases where the gain is both significant and predictable
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