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Testing ORDER BY speed
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To test a single factor X, it's necessary to keep all other factors equal while rerunning SQL statements with different values of X We tried First we created a table containing CHAR columns We populated the table using a randomizer that generated equal amounts of the letters A through G, with the result that some columns contained duplicate values or at least began with the same letters Filler columns were added as needed to ensure that total row length was the same in all tests Then we ran these three tests: Test #1, result shown in Figure 3-1 Number of rows: Varying from 1,000 to 10,000 Number of columns: Fixed at one Column length: Fixed at CHAR(10) SQL statements: Fixed at one:
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SELECT * FROM Table1 ORDER BY column1
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Test #2, result shown in Figure 3-2 Number of rows: Fixed at 1,000 Number of columns: Varying from one to ten Column length: Fixed at CHAR(10)
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SQL statements: Varying from one to ten:
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SELECT * FROM Table1 ORDER BY column1 SELECT * FROM Table1 ORDER BY column1, column2 SELECT * FROM Table1 ORDER BY column1, column2, column3
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etc Test #3, result shown in Figure 3-3 Number of rows: Fixed at 1,000 Number of columns: Fixed at one Column length: Varying from CHAR(1) to CHAR(10) SQL statements: Fixed at one:
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The timing numbers shown in Figures 3-1, 3-2, and 3-3 are the result and are averages for the Big Eight Tests were run on a single-processor machine using whatever memory the DBMS chose for its default configuration It's no surprise that an increase in the row count has a geometric effect if you increase the number of rows by a factor of ten, the job takes about twenty times as long to complete We had hoped for a better result That's achievable by adding more processors and more memory, because DBMSs can sort groups of tags in parallel threads and will thresh less if all tags are in memory Surprisingly, an increase in the column count has a more drastic effect than an increase in the column length Remember, total number of bytes was the same in both cases, so the effect is more than illusion The likely explanation is that our DBMSs are using multipass algorithms, comparing only one column at a time The bottom line: Take drastic action to reduce row count for example, by selecting only parts of a table at a time Take severe action to reduce column count for example, by concatenating two columns instead of specifying them separately Take moderate action to reduce column length for example, by using the SUBSTRING function It's also slightly helpful if values are presorted and unique on the first few characters, but Christmas comes only one day a year We make several more statements about what happens with a sort in the following sections
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Partial duplicates slow sorts
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Suppose you have a table with a CHAR column Populate the column with data in which the first five characters are duplicates Now SELECT:
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Delete all rows and repopulate, this time with random data SELECT again:
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SELECT column1 FROM Table1 ORDER BY column1 GAIN: 5/8
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Skipping the duplicated characters made the sort faster for five DBMSs
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Presorting speeds sorts
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Go back to your table with a CHAR column Populate the column with random data and SELECT:
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SELECT column1 FROM Table1 ORDER BY column1
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Delete all rows and repopulate, this time with data that's in alphabetic order and SELECT:
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SELECT column1 FROM Table1 ORDER BY column1 GAIN: 4/8
Having the rows already in order made the sort faster for half of the Big Eight
It's the defined length that matters
Remember that a variable-length column has a defined length and an actual length For example, a VARCHAR(30) column may contain ABC, in which case its defined length is 30 characters and its actual length is three characters For sorting, it's the 30-character definition that matters! We tested this by varying the defined length of a VARCHAR column, while keeping the actual data length the same The result was that Scenario #1 is slower than Scenario #2 even when the contents are the same in both cases:
Scenario #1: CREATE TABLE Table1 ( column1 VARCHAR(100)) SELECT column1 FROM Table1 ORDER BY column1 Scenario #2: CREATE TABLE Table2 ( column1 VARCHAR(10)) SELECT column1 FROM Table2 ORDER BY column1 GAIN: 6/8
Moral Defining VARCHAR columns with "room to grow" degrades sorts The gain might seem surprising, but it's consistent with the way that most sort algorithms work: They allot a fixed memory buffer in advance, and the size of the buffer depends on the maximum anticipated key size