RE: Data warehouse
От | Ansley, Michael |
---|---|
Тема | RE: Data warehouse |
Дата | |
Msg-id | 1BF7C7482189D211B03F00805F8527F70ECFB9@S-NATH-EXCH2 обсуждение исходный текст |
Список | pgsql-general |
>> Herouth Maoz wrote: >> >> > Maybe I'm missing the point here, but it seems to me that if >> > you simply use indices not as key definitions but as query >> > accelerators (as in "index the living daylights out of"), then >> > you may as well define a separate index on each and every >> > field. Why do multiple-field indices in such a case? >> >> Why do multi-field indexes? Umm. How about to avoid doing a sequential >> scan? If your query depends on multiple fields then once you've found >> the set based on the first index you would have to sequential scan that >> subset based on the second condition, unless you have a multi-field >> index. I suspect that Herouth was being a little facetious here. The bottom line is that multi-column indices are necessary for indices other than the primary key, and shouldn't be limited to seven fields. However, in creating a database that can handle more than 7 fields in indices, there is some overhead built in. Thus, it should be something that the administrator can decide. There are normally quite special reasons for wanting more than 7 fields in an index, and if you make it a pain to add the functionality to a PGSQL server, then people will rather design their dbs properly. However, in those cases where it is absolutely necessary, the functionality will at least be available. And I daresay that this option will be used when compiling a server to handle warehouses. MikeA
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