[GENERAL] correct example of a functional index usage?

[Dennis Gearon]

Is the following example a good, and correct one, for using a functional 
index? (the 'flip_bits' function would have to be written, and for the 
correct size of bit(N) ):

   I think I came up with a better way to search through some dates. It 
would use a functional index. I am trying to see if any of some 
appointments fall between two dates.

To explain this, think of a 16 day long year, represented by the binary 
bits in two bytes of a bit string

       the_table rows: 
           year, date_mask, appointment_title
            int, bit(16), text
           2004, B'11111000011110000'::bit(16), 'appointment_title'::text

Day 1 is on the left, day 16 is on the right. Say I wanted to find 
appointments in the year 2004 that happened between day 2 and day 9. One 
way that would find them is to search for all records like above that 
produced a non zero result when the day mask was ANDed against 
B'0111111110000000' and year = 2004:

   SELECT appointment
   FROM the_table
   WHERE 0 != (date_mask && B'0111111110000000')
       AND
                 year = 2004;
. This compares 9 dates in one operation, VERY fast. Except that it will 
not use an index - comparing dates alone would, even though there would 
be 16 times more rows to search, it'd be faster.

However, if I make two indexes, one on the standard field value, 
B'1111000011110000',

        CREATE INDEX norm_date_mask ON the_table(date_mask);
and the other on the bit reversed value, B'00001111000011110',
        CREATE INDEX flipped_date_mask ON flip_bits(the_table(date_mask));
now I can eliminate all those outside of the date ranges in another, 
indexed way like so.

date_range = B'0111111110000000';
flipped_date_range = flip_bits( B'0111111110000000');
the query looks like this:
SELECT appointment
FROM the_table
WHERE date_mask < date_range
   AND
            flipped_date < flip_bits(the_table(date_mask));
Now, I believe indexes are used for BOTH comparisons, and it will be a 
indexed, fast query, not a total search of the whole table. Plus, with a 
365/6 day sized date mask, 365/6 dates are searched in two operations.

If I needed SPECIFIC dates only, I could do the above search, and then 
add another AND condition that did the original AND against a smaller set.

I will experiment with this,but I'm pretty sure that I'm right.
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