[MI400] index keys techniques

["Steve Richter" <srichter@xxxxxxxxxxxxx>]

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An index orders its entries based on the byte value of the key section of the 
index entry.

No provision ( that I know of ) is made for descending keys.  Also, signed keys 
can be a bit suprising, as a BIN(2) fld that has the value of -1 is ordered 
after all positive values ( X'FFFF' is byte wise greater than X'0001' ).

Here is a technique I have come up with for handling descending keys: Use the 
NEG instruction to negate the value of the key fld before writing it to the 
index.  To chain to an index with a descending key, NEG the key before using 
the FNDINXEN instruction.

To order signed keys, preceed the bytes of the key with a single "sign" 
character. Positive key values set the sign char to a "P", negative values set 
the sign to "N". This way the negative key value will always sort before a 
positive value.

Steve Richter



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