Re: Building an index for next/last in a photo album.

[davidj]

Sounds rather complicated to me. If it's not too much of a hassle, I would suggest that you redesign your database to use img_id as the primary key (assuming that it is unique). You could then easily calculate the before and after img_id values and select them from the database in 1 SQL query. (It would also be very quick since primary keys are indexed)

my ($img_id) = $str =~ m/img_id=(\d+)/;
my $prev_id = $img_id - 1;
my $next_id = $img_id + 1;

Then the SQL query (something like):

"select WHATEVER from TABLE where img_id in ($prev_id, $img_id, $next_
+id)"

Hope this helps,
davidj

[Seumas]

The problem is that this application will serve infinite users with infinite albums, so while img_id is unique and sequential, it doesn't gaurantee that any two images belong to the same person or album.

[davidj]

Ok, this may or may not be a good solution, depending you whether or not you want to mess with your database design, but it will work.

Somewhere in the table put another column for a sequence number. So, instead of


img_id | album_id | uid |     name     | ext  
-------+----------+-----+--------------+------
    38 |      14  |  89 | mypetrat     | jpg  
    39 |      14  |  89 | mypetlemur   | jpg  
    40 |       2  |  12 | vacation_01  | jpg  
    41 |      14  |  89 | mypetvulture | jpg  
    42 |       2  |  12 | vacation_02  | jpg  
    43 |       2  |  12 | vacation_03  | jpg  
    44 |      14  |  89 | mypetcow     | jpg

you have something like


img_id | album_id | seq_id | uid |     name     | ext  
-------+----------+--------+-----+--------------+------
    38 |      14  |      1 |  89 | mypetrat     | jpg  
    39 |      14  |      2 |  89 | mypetlemur   | jpg  
    40 |       2  |      1 |  12 | vacation_01  | jpg  
    41 |      14  |      3 |  89 | mypetvulture | jpg  
    42 |       2  |      2 |  12 | vacation_02  | jpg  
    43 |       2  |      3 |  12 | vacation_03  | jpg  
    44 |      14  |      4 |  89 | mypetcow     | jpg

You could then grab the seq_id to the corresponding img_id

select seq_id from TABLE where img_id = $img_id

Then 1) calculate prev and next seq_id and 2) grab the img_ids you want be referencing the seq_id

my $p_seq_id = $seq_id - 1;
my $n_seq_id = $seq_id + 1;

"select img_id from TABLE where seq_id in ($p_seq_id, $seq_id, $n_seq_
+id)"

You could easily keep the current album seq_id value in another table and reference it when adding new images. As I said, it may be a hassle modifying your database, but it may be worthwhile in the long run as it more easily does what you want than the other options.

Note: This may be a good "teachable moment" as educators like to call them.

Perhaps the most important thing in designing an application that uses a database is to know thoroughly what you want from the application before you even begin designing the database. Too often, people design a database based on a less than thorough examination of the application requirements and end up having to redo it (the database.) :)

davidj