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Next: Modifying a table Up: NOTE 199 - Table Query Language Previous: Expressions

Subsections


Some further remarks

Time/position considerations

A position constant (e.g. 3h4m) is converted to radians, so it can be used easily in functions like SIN. Since a time constant has exactly the same format, it is also converted to radians, while the user may expect it to be expressed in seconds. The user has to take this into account if times are used in a comparison. For example, timecol > 3h4m is only correct if timecol has unit radians.
To make life easier the function TIME results in a value in radians, so it can be used directly in a comparison. For example, TIME(datecolumn) > 3h4m.
In the future this may change if units are handled in their full glory.

Optimization

A lot of development work can be done to improve the query optimization. At this stage a few optimizations are done.

The user can optimize a query by specifying the expression carefully. When using operator | | or &&, attention should be paid to the contents of the left and right branches. Both operators evaluate the right branch only if needed, so if possible the left branch should be the shortest one, i.e., the fastest to evaluate.

The user should also use functions, operators, and subqueries in a careful way.


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Next: Modifying a table Up: NOTE 199 - Table Query Language Previous: Expressions   Contents
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