It is difficult to find a complete explanation for Rushmore nowadays, from what I remember, it would create a bitmap where each bit represented the nth record of the table you wanted to search, then with a single, fast sequential scan it would set the nth bit to 1 if the record satisfied all clauses of your search, 0 otherwise.
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Bitmap Scan sounds a lot like Rushmore technology in Foxpro[1]. Are they the same?
1) https://en.wikipedia.org/wiki/FoxPro
It is difficult to find a complete explanation for Rushmore nowadays, from what I remember, it would create a bitmap where each bit represented the nth record of the table you wanted to search, then with a single, fast sequential scan it would set the nth bit to 1 if the record satisfied all clauses of your search, 0 otherwise.
Try to see if this makes any sense to you: http://www.foxpert.com/docs/howfoxproworks.en.htm