How many letters should be typed in “guessing” ajax search before first suggestion?

Question

When searching with ajax based suggesting field, how many letters should be typed before it starts to suggest results? 1,2,3..

Answer 1

Secondary rank

If you have another factor that can rank the results (e.g. "popularity", "kuakeli rank"):

• start with the first letter
• limit results by second rank, by picking the TOP N

(roughly: SELECT TOP 10 FROM Stuff WHERE Name LIKE "x*" ORDER BY Rank)

If you don't have that rank, you could still start after the first letter, but results aren't as useful. Still, there's no usability reason to start with a sorted list:

SELECT TOP 10 FROM Stuff WHERE Name LIKE "x*" ORDER BY Name

Performance reasons

Thee are a few traditional reasons for delaying the query:

• Part of your query runs synchronous, and you have to limit UI blocking
• You have to limit traffic / hits to the search provider

This is typically achieved by starting search if no key was pressed for the last few hundred milliseconds, or starting only after a minimum string length.

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There's one case left (that I can think of) where starting after the N th key might improve usability: Misleading matches. If your search pool contains elements like this:

SDBXXFDR 1722 Lengthy
SDBXXFDR 1723 Still Lengthy
...   
SDBXXFDR 1799 Short
SOBXXFDR 1972 Lengthy with ripples

And you just type "S", the "SD" results may hide the SO one, the chance of pickign the wrong one is rather high. However, that sems very constructed, and I'm not sure if incremental search is a good solution at all if this would be a problem.