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Is it usefull to have an ascending and a descending index on the same column?

asked 20 May '11, 12:34

Martin's gravatar image

accept rate: 14%

edited 21 May '11, 17:52

Volker%20Barth's gravatar image

Volker Barth

No, there is no need to define both an ascending column index and a descending index on the column. One or the other is sufficient and the SQL Anywhere server will perform a reverse index scan in cases that it makes sense to do so.

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answered 20 May '11, 13:17

Mark%20Culp's gravatar image

Mark Culp
accept rate: 41%

Just to add to Mark's sound answer (though Martin seems to refer to a single-column index):

The situation would be different if the index is a composite one (i.e. is built over 2 or more columns). Then not only the columns's order in the index (colA, colB vs. colB, colA), but also their according sort order will make a difference (colA ASC, colB ASC vs. colA ASC, colB DESC).

Cf. the docs.

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answered 21 May '11, 17:44

Volker%20Barth's gravatar image

Volker Barth
accept rate: 34%

edited 21 May '11, 17:52

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question asked: 20 May '11, 12:34

question was seen: 2,888 times

last updated: 21 May '11, 17:52