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VLDB SQL optimization tips

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SQL Generation and Data Processing: VLDB Properties » Details for all VLDB properties » Selecting and inserting data with SQL: Select/Insert » Attribute Selection and Form Selection Option for Intermediate Passes
Attribute Selection and Form Selection Option for Intermediate Passes

Normally, the MicroStrategy SQL Engine selects the minimum number of columns that are needed in each pass. For an intermediate pass, the SQL Engine usually only selects attribute ID forms. The SQL Engine also selects the attributes necessary to make the join, usually key attributes. Then in the final pass, additional attributes or attribute forms that are necessary for report display can be joined.

This algorithm is optimal in most cases, as it minimizes the size of intermediate tables. However, in certain schemas, especially denormalized ones, and schemas that use fact tables as both lookup tables and relationship tables, such an algorithm may cause additional joins in the final pass.

Example

A report template contains the attributes Region and Store, and metrics M1 and M2. M1 uses the fact table FT1, which contains Store_ID, Store_Desc, Region_ID, Region_Desc, and f1. M2 uses the fact table FT2, which contains Store_ID, Store_Desc, Region_ID, Region_Desc, and F2. With the normal SQL Engine algorithm, the intermediate pass that calculates M1 selects Store_ID and F1, the intermediate pass that calculates M2 selects Store_ID and F2. Then the final pass joins these two intermediate tables together. But that is not enough. Since Region is on the template, it should join upward to the region level and find the Region_Desc form. This can be done by joining either FT1 or FT2 in the final pass. So with the original algorithm, either FT1 or FT2 is being accessed twice. If these tables are big, and they usually are, the performance can be very slow. On the other hand, if Store_ID, Store_Desc, Region_ID, and Region_Desc are picked up in the intermediate passes, there is no need to join FT1 or FT2 does not need to be joined in the final pass, thus boosting performance.

For this reason, the following two properties are available in MicroStrategy:

• Attribute Form Selection Option for Intermediate Pass
• Attribute Selection Option for Intermediate Pass
• These properties intend to use bigger (wider) intermediate tables to save additional joins in the final pass and exchange space for time.
• These two properties work independently. One does not influence the other.
• Each property has two values. The default behavior is the original algorithm.
• When the property is enabled:
— The SQL Engine selects additional attributes or attribute forms in the intermediate pass, when they are directly available.
— The SQL Engine does not join additional tables to select more attributes or forms. So for intermediate passes, the number of tables to be joined is the same as when the property is disabled.

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