There's no reason to use IDX indexes whatsoever. For permanent indexes you should use compound structural index (CDX ). The compound structural index is opened automatically when table is open so you don't have to worry about it getting out of sync with data. The compound non-structural index are used for temporary indexes, if you ever need such.
The bottom line, use compound structural indexes.

Create table foo (f1 c(20),f2 D)
* Create 2 tags in structural index
index on f1 tag f1
index on f2 tag f2
Select * from foo where f1 = padr('asd',20) into .....

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>Can u please tell me where to use what? I am getting it a lot confused.
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>1) Does the Use command atomatically open the .idx ?? or does it have to be set implesetly i.e. with the set index to command ??
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>2) If the set index command is used then is the automatically opened cdx is thrown in the dustbin ??
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>3) DOes the sQL statements use the .idx formed ??
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>suppose
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>Create table foo (f1 c(20),f2 D)
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>index on f1 tag f1
>index on f2 tag f2
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>index on f1 to idxf1
>index on f2 to idxf2
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>use
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>And then
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>Select * from foo where f1 = padr('asd',20) into .....
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>does that use the cdx or idx.
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>If no cdx is present and only idx is present does the sql use it ??
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>How should i go about this.???
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>