How to list the attributes of a relation?
Why doesn't "join (relation1) (relation2)" work (as expected)?
Why are temporary relations always deleted?
How can I reduce the space wasted in a relation?
How do I update and delete tuples in a relation?

How do I compile in the readline libraries?
How do I enable/disable debugging?

How do I create the standard database set?
How do I apply a patch?

How much does LEAP cost?
Is there a Windows version?

SQL, ODBC and JDBC

Documentation Issues

Where can I find out more information about LEAP?

Subscribing/Unsubscribing
Posting

How to list the attributes of a relation?

Use the 'describe' command, eg. 'describe leaprel' returns:
Type Sz. Name
----------- --- ----------
STRING 25 NAME
STRING 25 FNAME
BOOLEAN 1 TEMP
INTEGER 4 NOATTRIBS
BOOLEAN 1 UPDATED
BOOLEAN 1 SYSTEM
Relation leaprel returned.

Why doesn't 'join (relation1) (relation2)' operator work (as expected)?

Joins require conditions. Fundamentally a join is a product followed by a select. The full form of a join is thus:
join (rel) (rel) (condition)

LEAP will (normally) look in two places for a condition - the first is the command line, and each expression must by enclosed in brackets. Values must be enclosed in quotes, eg.

join (rel1) (rel2) (rel1.attrib=rel2.attrib)
join (rel1) (rel2) ((rel1.attrib=rel2.attrib)and(rel2.attrib='2'))

If a condition is not specified, LEAP will look in the relship relation, which has eight attributes:

frelation - 'foreign' relation
prelation - 'primary' relation
fkey[1-3] - 'foreign' relation attributes 1, 2 and 3
pkey[1-3] - 'primary' relation attributes 1, 2 and 3

Essentially, specify the primary key and it's attributes, and the foreign key and it's attributes. For example, the LEAP system relation 'leapattributes' contains an attribute 'RELATION' which is a relation name. This is the foreign key, to the primary key 'NAME' in the relation 'leaprel'.

'leapattributes' defines all the attributes in a relation and 'leaprel' defines all the relations in a database.

By referencing the relship table, the join operator can derive the condition automatically, ie.

((leaprel.NAME=leapattributes.RELATION))

There is a third method LEAP will use, and this occurs only if no condition can be derived from the command line, or the productjoin status is set on (or --product-join is specified on the shell command prompt). In this scenario, LEAP will emulate SQL behaviour, which is to perform a simple 'PRODUCT' on the two relations specified (ie. the product, without the join).

Why are temporary relations always deleted?

Temporary relations are (by default) always deleted to remove the clutter from the database. If this is not acceptable, you can do one of two things:

Set the temporary setting 'off' - temporary indicates whether temporary relations are to be deleted. By default it is on.
Change the status - 'change relation' - This will update the temporary status of a relation such that it is a permanent relation.

How can I reduce the space wasted in a relation?

Relations can contain a great deal of wasted space. This is caused by the update and delete operators, which only mark tuples as deleted - the space isn't reclaimed. New to LEAP 1.2.5 is the 'compact' operator - which will remove this space. The worst offender is invariable 'leaprel' or 'leapattributes'. You can see the contents of a relation (including the 'deleted' tuples) by running the 'dump' command (or the dumprel program at the command line). To compress the space:

compact (relation)

Any errors encountered will be reported. In later releases of LEAP, the compression of wasted space will become an automated feature on shutdown.

How do I update and delete tuples in a relationship?

LEAP now allows existing tuples to be easily updated and deleted.
Deleting tuples takes the form: delete (relation) (condition)

This will search the relation specified for any tuples that match the specified condition. On encountering such a tuple, the tuple is marked as deleted. The condition takes the form of a condition you'd use in the select or join operator, and can refer to attributes in the relation, or to always true (or false) statements if required.

To delete an ENTIRE relation use the 'delrel' command. Deleting all of the tuples in a relation will leave an empty relation.

Updating tuples takes the form: update (relation) (condition) (update_statements)

The first three components of the update command are identical to the delete operator. The last component is new - This is where the attributes to be updated are specified, along with the values to update them with, eg.

update (r) (a='4') (a='5')

The relation 'r' is updated, where attribute 'a' equals '4', and where this condition is true, set the attribute to '5'.

Its' possible to specify multiple updates, by seperating the statements with commas, eg.

update (r) (a='4') (b='1',c='2')

Current restrictions with the update operator are that you can only set values to explicit values - it's not currently possible to set it to values elsewhere in the updated tuple; It's also not possible to update it based on a condition or value crossing relations (as you can do in SQL). These shortcomings will be addressed in a later release.

How do I compile in the readline libraries?

LEAP supports the readline libraries if they are installed. The readline libraries allow command line history and command editing. Versions 2.0 and2.1 are known to work.
Unlike many other packages, the readline libraries are *NOT* included in the default LEAP default distribution. See http://www.gnu.org for a copy.

In order to compile LEAP with the readline libraries, run configure with --with-readline, and set CPPFLAGS to include the include PATH (and -I flag). Sun systems (and others) will also need LD_LIBRARY_PATH to be set:

eg. if the readline libraries are included in /usr/local/include (and you're using a bourne (sh) compatible shell:

cd src
./configure --with-readline
CPPFLAGS=-I/usr/local/include
export CPPFLAGS
make depend
make

csh users: setenv CPPFLAGS -I/usr/local/include

Sun users: LD_LIBRARY_PATH /usr/local/lib

Linking with incorrect library search path will fail. If LD_LIBRARY_PATH is not supported, you may need to edit LDFLAGS to include: -L<path> -
with <path> being the path to the libraries.

How do I enable/disable debugging?

Debugging requires a particular function (do_debug) to be called at various points in the code. Even when the debug level is 0 (no debugging), this stilleats CPU cycles. If you're not keen to have this time wasted, run configure with --disable-debug, and recompile. This option can be added with others.

NB - The CPU time used is not huge, certainly compared to disk i/o.

How do I create standard database set?

Normal installation of the basic database set (master, tempdb, user and a few example databases) can be achieved by typing make install - However, the WIndows distribution might require this to be done manually if long filenames were not created when uncompressing the distribution archive (Winzip is recommended rather than pkunzip).
The command to install LEAP is:

./leap --configure ../scripts/install.src

This is the command that is executed by 'make install' - It starts LEAP up and configures the databases with the various commands in the install.src script.

How do I apply a patch?

LEAP bug fixes are distributed with patches. If you're using a Un*x system, it's generally very easy to apply a patch, which saves having to download the complete distribution all over again. For example, to apply the 1.2.3 patch to the 1.2.2 distribution you previously downloaded:

Download the 1.2.3 patch
Save the patch to the base of the 1.2.2 installation
Unpackage the package (It's usually compressed with gzip), and rename it leap_patch_122_123
Run the patch command: patch -p1 < ./leap_patch_122_123

The patch command is a freely available piece of software, written by Larry Wall (of perl fame). A link is available on the LEAP web pages.

How much does LEAP cost?

LEAP is distributed under the terms of the GNU General Public License. This license should be carefully studied if you indend to make use of any part of LEAP. You're not required to pay the author anything to use LEAP. However, if you feel like it, do so, but it's entirely at your own discretion.

Is there a Windows version?

There is now a Windows 95/NT version, offering much of the standard functionality you expect in Windows - command line history, and adjustable window size and fonts. See the LEAP web page for full details.

SQL, ODBC and JDBC

Support for SQL was never included in the original design for LEAP. LEAP was designed as an educational tool for students of relational algebra. However, the students of database theory courses need to be prepared for what the real world demands - and as far as database query languages go, this is SQL.

Before talking about LEAP's aspirations, it's worth making it clear that there are currently plenty of other SQL based databases available under the GNU GPL that might be more appropriate for your purposes. Be careful to check what the license agreement is, and whether you want access to the source code.

LEAP will enventually include support for an SQL subset, but before this can be offered, some major restructurnig is necessary of the source code. LEAP's parser (the part of the software that translates a simple command into an internal memory structure) is not capable of flexibly supporting both the relational algebra, and SQL. The next major release of LEAP (1.3/1.4) will use lex/yacc to support the existing query language. Once this has been successfully implemented, and debugged, it should be a simple process to slot in the lex/yacc definition for SQL (writing it is a different matter). Some of the more advanced features of SQL will drive new operators/mechanisms in LEAP.

Consequently, LEAP does not currently support JDBC or ODBC connectivity, nor will it for some time to come.

Where can I find out more information about LEAP?

LEAP's User documentation is included in the normal distribution. The website contains a copy too.

For an overview of the LEAP architecture, the original LEAP design document is available from the Sourceforge download page - This is the submitted project write up of the original Pascal version, which has been significantly superseded by the current version.

A book has recently been released, which includes a chapter on LEAP. "Theory and Practice of Relational Databases", 2nd Edition, by Stefan Stanczyk. Bob Champion and Richard Leyton, Routledge, 2001. For more detail, see the books accompanying website at http://www.theorypractice.org

There is a mailing list for all aspects of LEAP. When LEAP is installed it will offer to subscribe you to it. It's low volume (it's primarily announcement based, but other postings do appear). As of January 1999, it a fully moderated list. This has become necessary due to the high volume of spam (unsolicited, and unrelated e-mail) that has been appearing on the list. Hopefully it will be possible to return to an unmoderated list should the volume of spam decrease.

Subscribing/Unsubscribing

To subscribe or unsubscribe, visit http://lists.sourceforge.net/lists/listinfo/leap-announce

Posting to the List

You must first subscribe to the mailing list. Send an e-mail to leap-announce@lists.sourceforge.net - note that the list is moderated, and your post needs to be approved by the list moderator first. The moderator is the LEAP author, Richard Leyton.