What does LIKE '%%' actually do in a WHERE clause?

a developer is asking me what the exact function of "LIKE '%%'" in a WHERE clause is. I'm stumped, and cannot find anything on Google or in the Oracle docs.

The puzzler is this:

There is a SELECT on 4 tables using a cartesian join of all 4 and returning only the first 100 rows:

SELECT A.*, B.*, C.*, D.*
FROM A, B, C, D
WHERE ...
AND ROWNUM < 101;

The WHERE clause simply joins the 4 tables, there is no actual record selection.

As expected, this takes a wee while and results in a plan that hash joins the full table scan results for all tables.

Now, the developer has added to the where clause something like the following:

AND a.id like '%%' 
AND b.id like '%%' 
AND c.id like '%%' 
AND d.id like '%%'

This time, the query runs in seconds and uses indexes on the ID columns of all the tables.

The cardinality comes down to 1 for each table in the plan, as opposed to the previous 3 million or greater, but the results are exactly the same.

If '%%' is replaced by '%' it goes back to the original plan of hash joining the 4 full table spaces.

Google seems to ignore the %% in a search as does Oracle, whether in single or double quotes.

It's puzzling me (and a colleague) and we don't like being puzzled. All the developer knows is that he heard about this "trick" somewhere, but he can't remember where and he wants to know how it works and what it is doing!

Cheers,
Norm.

Norman Dunbar
Contract Senior Oracle DBA
Capgemini Database Build Team
Internal : 7 28 2051
External : 0113 231 2051

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