Parting Shots


We conducted an experiment using these two slightly different table definitions:

 CREATE TABLE Table1 (    column1 INTEGER NOT NULL,    column2 CHAR(64) NOT NULL,    column3 INTEGER NOT NULL,    column4 INTEGER NOT NULL) CREATE TABLE Table2 (    column1 DECIMAL(3),    column2 CHAR(64) NOT NULL,    column3 INTEGER NOT NULL,    column4 INTEGER NOT NULL) 

The purpose of the experiment was to see whether the two definitions for column1 INTEGER NOT NULL and DECIMAL(3)would make a difference to performance, especially with Microsoft. We thought it would. Our reasoning was:

  • The fact that column1 is the first column in the table is irrelevant if it's DECIMAL(3), because Microsoft will make it a variable-length column and thereforesince Microsoft shifts variable-length columns to the end of the rowthe column order for Table2 will actually be {column2, column3, column4, column1} .

  • The fact that Table2.column1 is DECIMAL(3) means it will be stored as a three-byte string.

We weren't testing to see whether operations on column1 would go faster. Instead we were testing to see whether accesses involving column3 would go faster! Our suspicion was that column3 would be misaligned . On an Intel machine, if (offset within page) is not divisible by four, there is a penalty for reading a four-byte integer. So we inserted 10,000 rows into both tables, then scanned for column3 = 51700 (which was always false ). Andas expectedthe scan worked faster on Table1 , where column1 was defined as INTEGER NOT NULL (GAIN: 4/8).

Data type does make a difference!


SQL Performance Tuning
SQL Performance Tuning
ISBN: 0201791692
EAN: 2147483647
Year: 2005
Pages: 125

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