Once created, a database schema can be updated, to add or remove attributes, add or remove classes or schema, add indexes or contraints.
To introduce the necessity of indexes, we propose to perform the following operations:
?for (x in 1 <= 50000) new Person(firstname : "xx" + string(x));?select Person.firstname = "xx20";= bag(23336.2.420154:oid) ?select Person.firstname = "xx10";= bag(23316.2.824639:oid)
The first operation creates 50000 person instances: as you can notice, this operation takes a few seconds. The two last operations query for person instances according to their firstname attribute. These operations also take a few seconds to perform and take a significant amount of CPU.
A good idea is to create an index on the attributes - for instance firstname, lastname and age - for which one wants to perform efficient queries.
The first step to add an index is to add the index specification to the class Person in the ODL schema person.odl as follows:
class Person {
string firstname;
char lastname;
int age;
Address addr;
...
set<Person *> children;
index on firstname;
index on lastname;
index on age;
};
The database schema can then be updated using the eyedbodl tool:
% eyedbodl -d foo -u person.odl
Updating 'person' schema in database foo...
Creating [NULL] hashindex 'index<type = hash, propagate = on> on Person.firstname' on class 'Person'...
Creating [NULL] hashindex 'index<type = hash, propagate = on> on Person.lastname' on class 'Person'...
Creating [NULL] btreeindex 'index<type = btree, propagate = on> on Person.age' on class 'Person'...
Done
Now, you can try again to query for Person instances according to its firstname, lastname or age, and you will notice that these operations are now immediate.:
%eyedboql -d foo -w?select Person.firstname = "xx20";= bag(23336.2.420154:oid) ?select Person.firstname = "xx10";= bag(23316.2.824639:oid)
In the same way, you can add a notnull and a unique constraint on the lastname attribute within the class Person.
First step is to add the constraint specification to the class Person in the person.odl file as follows:
class Person {
string firstname;
string lastname;
int age;
Address addr;
...
index on firstname;
index on lastname;
index on age;
constraint<notnull> on lastname;
constraint<unique> on lastname;
};
Then, use the eyedbodl tool to update the database schema:
% eyedbodl -d foo -u person.odl
Updating 'person' schema in database foo...
Creating [NULL] notnull_constraint 'constraint<notnull, propagate = on> on Person.lastname' on class 'Person'...
Creating [NULL] unique_constraint 'constraint<unique, propagate = on> on Person.lastname' on class 'Person'...
Done
Now try to create two person instances with the same lastname attribute:
%eyedboql -d foo -w?new Person(lastname : "curtis");= 79902.2.884935:oid ?new Person(lastname : "curtis");near line 2: 'new Person(lastname : "curtis")' => oql error: new operator 'new<oql$db> Person(lastname:"curtis"); ' : unique[] constraint error: attribute path 'Person.lastname'.
or with no lastname attribute:
? new Person();
near line 3: 'new Person()' => oql error: new operator 'new<oql$db> Person(); ' : notnull[] constraint error: attribute path 'Person.lastname'.
It is possible to remove a class in a schema using eyedbodl. For instance, to remove the class Employee in the already introduced schema:
% eyedbodl -d foo -u --rmcls=Employee
Updating 'UNTITLED' schema in database foo...
Removing class Employee
Done
You can then check the class removal by:
% eyedbodl -d foo --gencode=ODL
//
// EyeDB Version 2.8.4 Copyright (c) 1995-2007 SYSRA
//
// UNTITLED Schema
//
// Generated by eyedbodl at Mon Dec 10 13:44:03 CET 2007
//
class Address (implementation <hash, hints = "key_count = 2048;">) {
attribute int32 num;
attribute string street;
attribute string town;
attribute string country;
};
class Person (implementation <hash, hints = "key_count = 2048;">) {
attribute string firstname;
attribute string lastname;
attribute int32 age;
attribute Address addr;
relationship Person* spouse inverse Person::spouse;
attribute set<Person*> children;
index<type = hash, hints = "key_count = 4096; initial_size = 4096; extend_coef = 1; size_max = 4096;", propagate = on> on Person.firstname;
index<type = hash, hints = "key_count = 4096; initial_size = 4096; extend_coef = 1; size_max = 4096;", propagate = on> on Person.lastname;
constraint<unique, propagate = on> on Person.lastname;
constraint<notnull, propagate = on> on Person.lastname;
index<type = btree, hints = "degree = 128;", propagate = on> on Person.age;
};
It is as well possible to remove entirely the database schema:
% eyedbodl -d foo -u --rmsch
Updating 'UNTITLED' schema in database foo...
Removing [2570.2.500986:oid] hashindex 'index<type = hash, hints = "key_count = 4096; initial_size = 4096; extend_coef = 1; size_max = 4096;", propagate = on> on Person.firstname' from class 'Person'...
Removing [2585.2.286352:oid] hashindex 'index<type = hash, hints = "key_count = 4096; initial_size = 4096; extend_coef = 1; size_max = 4096;", propagate = on> on Person.lastname' from class 'Person'...
Removing [2599.2.7912:oid] btreeindex 'index<type = btree, hints = "degree = 128;", propagate = on> on Person.age' from class 'Person'...
Removing [2625.2.396262:oid] unique_constraint 'constraint<unique, propagate = on> on Person.lastname' from class 'Person'...
Removing [2620.2.240536:oid] notnull_constraint 'constraint<notnull, propagate = on> on Person.lastname' from class 'Person'...
Removing class Address
Removing class Person
Removing class set<Person*>
Done
The result can be checked with:
% eyedbodl -d foo --gencode=ODL
//
// EyeDB Version 2.8.4 Copyright (c) 1995-2007 SYSRA
//
// UNTITLED Schema
//
// Generated by eyedbodl at Mon Dec 10 13:44:03 CET 2007
//