Does a multi-column index work for single column selects too?

Question

I\'ve got (for example) an index:

CREATE INDEX someIndex ON orders (customer, date);

Does this index only accelerate queries where cust

Answer 1

I commonly use combined indexes to sort through data I wish to paginate or request "streamily".

Assuming a customer can make more than one order.. and customers 0 through 11 exist and there are several orders per customer all inserted in random order. I want to sort a query based on customer number followed by the date. You should sort the id field as well last to split sets where a customer has several identical dates (even if that may never happen).

sqlite> CREATE INDEX customer_asc_date_asc_index_asc ON orders
          (customer ASC, date ASC, id ASC);

Get page 1 of a sorted query (limited to 10 items):

sqlite> SELECT id, customer, date FROM orders
          ORDER BY customer ASC, date ASC, id ASC LIMIT 10;

2653|1|1303828585
2520|1|1303828713
2583|1|1303829785
1828|1|1303830446
1756|1|1303830540
1761|1|1303831506
2442|1|1303831705
2523|1|1303833761
2160|1|1303835195
2645|1|1303837524

Get the next page:

sqlite> SELECT id, customer, date FROM orders WHERE
          (customer = 1 AND date = 1303837524 and id > 2645) OR
          (customer = 1 AND date > 1303837524) OR
          (customer > 1)
          ORDER BY customer ASC, date ASC, id ASC LIMIT 10;

2515|1|1303837914
2370|1|1303839573
1898|1|1303840317
1546|1|1303842312
1889|1|1303843243
2439|1|1303843699
2167|1|1303849376
1544|1|1303850494
2247|1|1303850869
2108|1|1303853285

And so on...

Having the indexes in place reduces server side index scanning when you would otherwise use a query OFFSET coupled with a LIMIT. The query time gets longer and the drives seek harder the higher the offset goes. Using this method eliminates that.

Using this method is advised if you plan on joining data later but only need a limited set of data per request. Join against a SUBSELECT as described above to reduce memory overhead for large tables.