It seems to believe that the number of rows returned for the >49.999
case will be 4 times the number for the >50 case. If that was true, then
the sequential scan would be correct.
ALTER TABLE <table> ALTER COLUMN diagonalsize SET STATISTICS 1000;
ANALZYE <table>;
Send back EXPLAIN ANALYZE output for the >49.999 case.
> The query plan for diagonalSize > 50.000 is ...
>
> Index Scan using nrgfeature_xys_index on nrgfeature f
> (cost=0.00..17395.79 rows=4618 width=220)
> Index Cond: ((upperrightx > 321264.236977215::double precision) AND
> (lowerleftx < 324046.799812083::double precision) AND (upperrighty >
> 123286.261898636::double precision) AND (lowerlefty <
> 124985.927450476::double precision) AND (diagonalsize > 50::double
> precision))
>
> ... while for diagonalSize > 49.999 is ...
>
> Seq Scan on nrgfeature f (cost=0.00..31954.70 rows=18732 width=220)
> Filter: ((upperrightx > 321264.236977215::double precision) AND
> (lowerleftx < 324046.799812083::double precision) AND (upperrighty >
> 123286.261898636::double precision) AND (lowerlefty <
> 124985.927450476::double precision) AND (diagonalsize > 49.999::double
> precision))