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I did any performance tests on my system and I have to notice that all queries with String constrains are very slow.
For the whole example is the index true;
 

In my testcase an normal query like:

        Query query=db.query();
        query.constrain(Personal.class);
        query.descend("name").constrain("Mustermann");

deliver this results.

    Benchmark 6  (Checksumme: 189)--> 342 ms        // first round and open DB, all other rounds run with opened DB
    Benchmark 6  (Checksumme: 189)--> 76 ms
    Benchmark 6  (Checksumme: 189)--> 95 ms
    Benchmark 6  (Checksumme: 189)--> 16 ms
    Benchmark 6  (Checksumme: 189)--> 15 ms
    Benchmark 6  (Checksumme: 189)--> 74 ms
    Benchmark 6  (Checksumme: 189)--> 21 ms

But this query is much slower as befor.

    Query query=db.query();
    query.constrain(Personal.class);
    query.descend("name").constrain("Mustermann").startsWith(true);

No matter which string constraint I have used, the query is slow.

    Benchmark 6  (Checksumme: 201)--> 550 ms       // first round and open DB, all other rounds run with opened DB
    Benchmark 6  (Checksumme: 201)--> 207 ms
    Benchmark 6  (Checksumme: 201)--> 207 ms
    Benchmark 6  (Checksumme: 201)--> 203 ms
    Benchmark 6  (Checksumme: 201)--> 206 ms
    Benchmark 6  (Checksumme: 201)--> 217 ms
    Benchmark 6  (Checksumme: 201)--> 203 ms

Why are queries with (startswith, endswith, contains, like, ...) so much slower????

Can anybody give an advice?  

At an other query the penaty is much bigger.

without startswith(false)

    Benchmark 7  (Checksumme: 3)--> 80 ms
    Benchmark 7  (Checksumme: 3)--> 3 ms
    Benchmark 7  (Checksumme: 3)--> 3 ms
    Benchmark 7  (Checksumme: 3)--> 2 ms
    Benchmark 7  (Checksumme: 3)--> 3 ms
    Benchmark 7  (Checksumme: 3)--> 2 ms
    Benchmark 7  (Checksumme: 3)--> 2 ms

with startswith(false); (same objectcount) 

    Benchmark 7  (Checksumme: 3)--> 668 ms
    Benchmark 7  (Checksumme: 3)--> 586 ms
    Benchmark 7  (Checksumme: 3)--> 582 ms
    Benchmark 7  (Checksumme: 3)--> 580 ms
    Benchmark 7  (Checksumme: 3)--> 574 ms
    Benchmark 7  (Checksumme: 3)--> 569 ms
    Benchmark 7  (Checksumme: 3)--> 745 ms

It cant be! I look in to the source code and find in the db4o-7.4\src\db4oj\core\src\com\db4o\internal\query\processor\QEStringCmp file the following line. Should this mean that all string comperators dont support an index?

public boolean supportsIndex(){
        return false;
}

Origon45:

Why are queries with (startswith, endswith, contains, like, ...) so much slower????
Can anybody give an advice?  

More than likely, these queries cannot use the index. The db needs to walk through all the objects and evaluate your predicate against them.
I'm not sure that an index for these queries makes any sense. If you give more details about your application maybe there are other ways to solve the problem?

Erik.