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x_mapping
xkey min_x  max_x
...
ykey min_y  max_y
1 -9999 2.5  1
2 2.5 3.5

  my $xkey = $dbh->selectrow_array("SELECT xkey FROM x_mapping WHERE m
+in_x <= ? AND ? < max_x", {}, $x, $x );
  my $ykey = $dbh->selectrow_array("SELECT ykey FROM y_mapping WHERE m
+in_y <= ? AND ? < max_y", {}, $y, $y );

x_mapping
xkey max_x
...

  my $xkey = $dbh->selectrow_array("SELECT xkey FROM x_mapping WHERE ?
+ < max_x ORDER BY max_x ASC LIMIT 1", {}, $x );
  my $ykey = $dbh->selectrow_array("SELECT ykey FROM y_mapping WHERE ?
+ < max_y ORDER BY max_y ASC LIMIT 1", {}, $y );