18 KiB
@defgroup myddas The MYDDAS Data-base interface
@ingroup packages @{ The MYDDAS database project was developed within a FCT project aiming at the development of a highly efficient deductive database system, based on the coupling of the MySQL relational database system with the YAP Prolog system. MYDDAS was later expanded to support the ODBC interface, postgres and sqlite3.
@}
@defgroup MYDDAS_ABI MYDDAS Predicates @ingroup myddas
[TOC]
@{
@pred db view(+,+,+). @pred db view(+,+).
If we import a database relation, such as an edge relation representing the edges of a directed graph, through
?- db_import('Edge',edge).
yes
sqliand we then write a query to retrieve all the direct cycles in the graph, such as
?- edge(A,B), edge(B,A).
A = 10,
B = 20 ?
this is clearly inefficient [3], because of relation-level
access. Relation-level access means that a separate SQL query will be
generated for every goal in the body of the clause. For the second
edge/2 goal, a SQL query is generated using the variable bindings that
result from the first edge/2 goal execution. If the second
edge/2 goal
fails, or if alternative solutions are demanded, backtracking access the
next tuple for the first edge/2 goal and another SQL query will be
generated for the second edge/2 goal. The generation of this large
number of queries and the communication overhead with the database
system for each of them, makes the relation-level approach inefficient.
To solve this problem the view level interface can be used for the
definition of rules whose bodies includes only imported database
predicates. One can use the view level interface through the predicates
db_view/3 and db_view/2:
?- db_view(Conn,PredName(Arg_1,...,Arg_n),DbGoal).
?- db_view(PredName(Arg_1,...,Arg_n),DbGoal).
All arguments are standard Prolog terms. Arg1 through Argn
define the attributes to be retrieved from the database, while
DbGoal defines the selection restrictions and join
conditions. Conn is the connection identifier, which again can be
dropped. Calling predicate PredName/n will retrieve database
tuples using a single SQL query generated for the DbGoal. We next show
an example of a view definition for the direct cycles discussed
above. Assuming the declaration:
?- db_import('Edge',edge).
yes
we write:
?- db_view(direct_cycle(A,B),(edge(A,B), edge(B,A))).
yes
?- direct_cycle(A,B)).
A = 10,
B = 20 ?
This call generates the SQL statement:
SELECT A.attr1 , A.attr2
FROM Edge A , Edge B
WHERE B.attr1 = A.attr2 AND B.attr2 = A.attr1;
Backtracking, as in relational level interface, can be used to retrieve the next row of the view.
The view interface also supports aggregate function predicates such as
sum, avg, count, min and max. For
instance:
?- db_view(count(X),(X is count(B, B^edge(10,B)))).
generates the query :
SELECT COUNT(A.attr2)
FROM Edge A WHERE A.attr1 = 10;
To know how to use db view/3, please refer to Draxler's Prolog to
SQL Compiler Manual.
@pred db_sql(+,+,?). @pred db_sql(+,?).
It is also possible to explicitly send a SQL query to the database server using
?- db_sql(Conn,SQL,List).
?- db_sql(SQL,List).
where SQL is an arbitrary SQL expression, and List is a list holding the first tuple of result set returned by the server. The result set can also be navigated through backtracking.
Example:
?- db_sql('SELECT * FROM phonebook',LA).
LA = ['D','John Doe',123456789] ?
@pred db_assert(+,+). @pred db_assert(+).
Assuming you have imported the related base table using
db_import/2 or db_import/3, you can insert to that table
by using db_assert/2 predicate any given fact.
?- db_assert(Conn,Fact).
?- db_assert(Fact).
The second argument must be declared with all of its arguments bound to
constants. For example assuming helloWorld is imported through
db_import/2:
?- db_import('Hello World',helloWorld).
yes
?- db_assert(helloWorld('A' ,'Ana',31)).
yes
This, would generate the following query
INSERT INTO helloWorld
VALUES ('A','Ana',3)
which would insert into the helloWorld, the following row:
A,Ana,31. If we want to insert NULL values into the
relation, we call db_assert/2 with a uninstantiated variable in
the data base imported predicate. For example, the following query on
the YAP-prolog system:
?- db_assert(helloWorld('A',NULL,31)).
yes
Would insert the row: A,null value,31 into the relation
Hello World, assuming that the second row allows null values.
@pred db insert(+,+,+). @pred db insert(+,+).
This predicate would create a new database predicate, which will insert any given tuple into the database.
?- db_insert(Conn,RelationName,PredName).
?- db_insert(RelationName,PredName).
This would create a new predicate with name PredName, that will
insert tuples into the relation RelationName. is the connection
identifier. For example, if we wanted to insert the new tuple
('A',null,31) into the relation Hello World, we do:
?- db_insert('Hello World',helloWorldInsert).
yes
?- helloWorldInsert('A',NULL,31).
yes
@pred db_get_attributes_types(+,+,?). otype for this predicate is the following:
?- db_get_attributes_types(Conn,RelationName,ListOfFields).
?- db_get_attributes_types(RelationName,ListOfFields).
You can use the
predicate db_get_attributes types/2 or db_get_attributes_types/3, to
know what are the names and attributes types of the fields of a given
relation. For example:
?- db_get_attributes_types(myddas,'Hello World',LA).
LA = ['Number',integer,'Name',string,'Letter',string] ?
yes
where Hello World is the name of the relation and myddas is the connection identifier.
@pred db_number_of_fields(+,?). @pred db_number_of_fields(+,+,?).
The prototype for this predicate is the following:
?- db_number_of_fields(Conn,RelationName,Arity).
?- db_number_of_fields(RelationName,Arity).
You can use the predicate db_number_of_fields/2 or
db_number_of_fields/3 to know what is the arity of a given
relation. Example:
?- db_number_of_fields(myddas,'Hello World',Arity).
Arity = 3 ?
yes
where Hello World is the name of the
relation and myddas is the connection identifier.
@pred db_datalog_describe(+,+). @pred db_datalog_describe(+).
The db datalog_describe/2 predicate does not really returns any
value. It simply prints to the screen the result of the MySQL describe
command, the same way as DESCRIBE in the MySQL prompt would.
?- db_datalog_describe(myddas,'Hello World').
+----------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------+------+-----+---------+-------+
+ Number | int(11) | YES | | NULL | |
+ Name | char(10) | YES | | NULL | |
+ Letter | char(1) | YES | | NULL | |
+----------+----------+------+-----+---------+-------+
yes
@pred db_describe(+,+).
@pred db_describe(+)
The db_describe/3 predicate does the same action as
db_datalog_describe/2 predicate but with one major
difference. The results are returned by backtracking. For example, the
last query:
?- db_describe(myddas,'Hello World',Term).
Term = tableInfo('Number',int(11),'YES','',null(0),'') ? ;
Term = tableInfo('Name',char(10),'YES','',null(1),'' ? ;
Term = tableInfo('Letter',char(1),'YES','',null(2),'') ? ;
no
@pred db_datalog_show_tables(+). @pred db_datalog_show_tables
If we need to know what relations exists in a given MySQL Schema, we can use
the db_datalog_show_tables/1 predicate. As db_datalog_describe/2,
it does not returns any value, but instead prints to the screen the result of the
SHOW TABLES command, the same way as it would be in the MySQL prompt.
?- db_datalog_show_tables(myddas).
+-----------------+
| Tables_in_guest |
+-----------------+
| Hello World |
+-----------------+
yes
@pred db_show_tables(+, ?).
@pred db_show_tables(?)
The db_show_tables/2 predicate does the same action as
db_show_tables/1 predicate but with one major difference. The
results are returned by backtracking. For example, given the last query:
?- db_show_tables(myddas,Table).
Table = table('Hello World') ? ;
no
@pred db_top_level(+,+,+,+,+). @pred db_top_level(+,+,+,+).
Through MYDDAS is also possible to access the MySQL Database Server, in the same wthe mysql client. In this mode, is possible to query the SQL server by just using the standard SQL language. This mode is exactly the same as different from the standard mysql client. We can use this mode, by invoking the db top level/5. as one of the following:
?- db_top_level(mysql,Connection,Host/Database,User,Password).
?- db_top_level(mysql,Connection,Host/Database/Port,User,Password).
?- db_top_level(mysql,Connection,Host/Database/UnixSocket,User,Password).
?- db_top_level(mysql,Connection,Host/Database/Port/UnixSocket,User,Password).
Usage is similar as the one described for the db_open/5 predicate discussed above. If the login is successful, automatically the prompt of the mysql client will be used. For example:
?- db_top_level(mysql,con1,localhost/guest_db,guest,'').
opens a
connection identified by the con1 atom, to an instance of a MySQL server
running on host localhost, using database guest db and user guest with
empty password. After this is possible to use MYDDAS as the mysql
client.
?- db_top_level(mysql,con1,localhost/guest_db,guest,'').
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Welcome to the MySQL monitor.
Commands end with ; or \g.
Your MySQL connection id is 4468 to server version: 4.0.20
Type 'help;' or '\h' for help.
Type '\c' to clear the buffer.
mysql> exit
Bye
yes
?-
@pred db_verbose(+).
When we ask a question to YAP, using a predicate asserted by
db_import/3, or by db_view/3, this will generate a SQL
QUERY. If we want to see that query, we must to this at a given
point in our session on YAP.
?- db_verbose(1).
yes
?-
If we want to
disable this feature, we must call the db_verbose/1 predicate with the value 0.
@pred db_top_level(+,+,+,+).
@pred db_module(?).
When we create a new database predicate, by using db_import/3,
db_view/3 or db_insert/3, that predicate will be asserted
by default on the user module. If we want to change this value, we can
use the db_module/1 predicate to do so.
?- db_module(lists).
yes
?-
By executing this predicate, all of the predicates asserted by the predicates enumerated earlier will created in the lists module. If we want to put back the value on default, we can manually put the value user. Example:
?- db_module(user).
yes
?-
We can also see in what module the predicates are being asserted by doing:
?- db_module(X).
X=user
yes
?-
@pred db_my_result_set(?).
The MySQL C API permits two modes for transferring the data generated by a query to the client, in our case YAP. The first mode, and the default mod-MySQL, is to store the result. This mode copies all the information generated to the client side.
?- db_my_result_set(X).
X=store_result
yes
The other mode that we can use is use result. This one uses the result set created directly from the server. If we want to use this mode, he simply do
?- db_my_result_set(use_result).
yes
After this command, all
of the database predicates will use use result by default. We can change
this by doing again db_my_result_set(store_result).
@pred db_my_sql_mode(+Conn,?SQL_Mode). @pred db_my_sql_mode(?SQL_Mode).
The MySQL server allows the user to change the SQL mode. This can be very useful for debugging proposes. For example, if we want MySQL server not to ignore the INSERT statement warnings and instead of taking action, report an error, we could use the following SQL mode.
?-db_my_sql_mode(traditional). yes
You can see the available SQL Modes at the MySQL homepage at http://www.mysql.org.
@}
@defgroup MYDDAS_INSTall Requirements and Installation Guide
@ingroup myddas
@{
Next, we describe how to use YAP plus the MYDDAS System. MYDDAS includes its own copy of SQLite3, a popular DBMS that is highky portable. It can also them interface to the MySQL development libraries, Postgres or the ODBC development libraries. At least one of the this development libraries must be installed on the computer system, otherwise MYDDAS will not compile. The MySQL development libraries from MySQL 3.23 an above are know to work. We recommend the usage of MySQL versus ODBC, but it is possible to have both options installed
At the same time, without any problem. The MYDDAS system automatically controls the two options. Currently, MYDDAS is know to compile without problems in Linux. The usage of this system on Windows has not been tested yet. MYDDAS must be enabled at configure time. This can be done with the following options:
+ `--enable-myddas`
This option will detect which development libraries are installed on the computer system, MySQL, ODBC or both, and will compile the Yap system with the support for which libraries it detects;
+ `--enable-myddas-stats`
This option is only available in MySQL. It includes code to get statistics from the MYDDAS system;
+ `--enable-top-level`
This option is only available in MySQL. It enables the option to interact with the MySQL server in two different ways. As if we were on the MySQL Client Shell, and as if we were using Datalog.
@}
@defgroup myddasarch MYDDAS Architecture @ingroup myddas
@{
The system includes four main blocks that are put together through the MYDDAS interface: the Yap Prolog compiler, the MySQL database system, an ODBC level and a Prolog to SQL compiler. Current effort is put on the MySQL interface rather than on the ODBC interface. If you want to use the full power of the MYDDAS interface we recommend you to use a MySQL database. Other databases, such as Oracle, PostGres or Microsoft SQL Server, can be interfaced through the ODBC layer, but with limited performance and features support.
The main structure of the MYDDAS interface is simple. Prolog queries involving database goals are translated to SQL using the Prolog to SQL compiler; then the SQL expression is sent to the database system, which returns the set of tuples satisfying the query; and finally those tuples are made available to the Prolog engine as terms. For recursive queries involving database goals, the YapTab tabling engine provides the necessary support for an efficient evaluation of such queries.
An important aspect of the MYDDAS interface is that for the programmer the use of predicates which are defined in database relations is completely transparent. An example of this transparent support is the Prolog cut operator, which has exactly the same behaviour from predicates defined in the Prolog program source code, or from predicates defined in database as relations.
Name = 'John Doe',
Number = 123456789 ?
yes
Backtracking can then be used to retrieve the next row of the relation phonebook. Records with particular field values may be selected in the same way as in Prolog. (In particular, no mode specification for database predicates is required). For instance:
?- phonebook(Letter,'John Doe',Letter).
Letter = 'D',
Number = 123456789 ?
yes
generates the query
SELECT A.Letter , 'John Doe' , A.Number
FROM 'phonebook' A
WHERE A.Name = 'John Doe';
@}