path: root/funtools/notes/view.notes

Views:

We can implement views as pseudo files that actually refer to a row in a
data base containing column, filter, and format specifiers for that view.

To create a view:

	funview -p "format" -r "filter" -c "columns" view file

This will make an entry into a data base having the form:

	view	file		columns		filter		format
	----	-----------	-----------	-----------	--------------

Data base name will be the first one found:

1. getenv("FUN_VIEWBASE")
2. ./.funtools.vu
3. ~/.funtools.vu

For example:

	funview -p "I=%3d" -r "cir 512 512 .1" -c "x y pha pi" myview snr.ev

adds to database:

	view	file		columns		filter		format
	----	-----------	-----------	-----------	--------------
	myview	snr.ev		x y pha pi	cir 512 512 .1	I=%.3d

Also:

	funview -d view		# delete view from database
	funview -l		# list views

Of course, you can edit the database file directly (but does this avoid
some needed checking?) ...  and we might also have a GUI ...

You then use the view id instead of a file in any funtools program,
prefixed by v:, i.e.

	fundisp v:myview
	funtable v:myview foo.fits

The first command will run fundisp on the associated file, but using
the specified filter, column, and format spec. The second runs funtable,
which has no need of the print format but still utilizes the filter
and column specifiers.

You can override any of the three view specifiers explicitly:

	fundisp v:myview'[cir 400 400 1]'

uses the view format and column spec, but overrides the filter, while:

	fundisp -f "I=%5d" v:myview'[cir 400 400 1]' "x y time"

overrides everything but the file name ... probably useless but at
least consistent.

The syntax of a view should support templates, so that the view filter,
columns, and format parameters can be used with multiple files.

For example:

	funview -p "I=%3d" -r "cir 512 512 .1" -c "x y pha pi" myview "*.ev"

adds to data base:

	view	file		columns		filter		format
	----	-----------	-----------	-----------	--------------
	myview	*.ev		x y pha pi	cir 512 512 .1	I=%.3d

You then explicitly use a template view for a file that matches the template
by specifying the actual filename instead of the view:

	fundisp v:foo.ev

Since foo.ev matches the *.ev template, this will use the myview
columns, filter, and format on the foo.ev data file.

Note possible conflict if a file name matches a view but ... we can
avoid this to some extent by not allowing view names to contain ".",
":", "/" (same sort of restriction in xpa).

A view GUI can have a selectable option to tell funtools programs
always to assume a file is a view, even if v: is not prepended. This means
that, if you start the GUI and select the "auto view" switch, then:

      fundisp v:myview
and
      fundisp myview

will be handled identically. Without the GUI running, the v: prefix will
be required.

There should be a way to specify which view to use for a given file:
	
	fundisp v(fi3):foo.fits

means use view fi3 on file foo.fits

	fundisp v(reg1,col2):foo.fits

means use views reg1 and col2 (in that order) on file foo.fits.

Wild idea: we can implement a join of files as a view (just like the
big boy databases).  I think we get this for free if FunOpen just
opens a pipe: pseudo file (which is implemented and might actually
work) instead of a disk file.

Example: we define a view called "myjoin" to be:

	view	file		...
	----	-----------	...
	myjoin	"funjoin -j1 key -j2 key2 cat1.fits cat2.fits" ...

where the implied output arg is stdout and then we can execute:

	fundisp v:myjoin

and get the joined result. View filters and columns would be applied to the
output and it all just might work ...

Good points:

Explicit specification of a pseudo-file "v:[viewid]" is unambiguous
and alleviates the need to rely on a separate process to prevent
accident use of a view.  If you don't want to use the view, just don't
specify it.

Easy flexibility for overriding view parameters.

Support for file templates allows default setups of filter, column,
and formats (can even use "*" as the file for a global setup).

Uses standard funtools file type paradigm (socket:, pipe:, shm: etc).

If we can implement arbitrary commands as a view file type via pipe:
(particularly funjoin), then our views will be similar (superset?) to
SQL views (see below).

Bad points:

The original idea of changing getenv would have made extensions to the
list of view parameters automatic. We now have to implement each
access of a view parameter explicitly, as opposed to having getenv do
it automatically.  Actually, this is a two-edged sword: it also could
be a bad thing that each and every getenv would look for a global value.

Design:

I. FunOpen("v:id", ....)	

To process a view file spec inside FunOpen():

1. Look for view file, either in default (~/.funtools.views) or
pointed to by FUN_VIEWFILE. If it does not exist, return ERROR.

2. If filespec contains a directory character ("/") go to 4.

3. ID or simple file: Strip off and save filter spec. Look for ID
name. If not found, go to 4. If found, replace input name with FILE
name. If no filter spec is specified, use FILTER, otherwise use
(override) filter spec. Save column spec for later use by
FunColumnActivate.

4. Pathname or Simple file: Try to match file spec again template
FILEs. If found, replace input name with FILE name. If no filter spec
is specified, use FILTER, otherwise use (override) filter spec. Save
column spec for later use by FunColumnActivate.

II. FunColumnActivate(fun, columns, plist)

1. If columns != NULL, use columns (might be override).

2. If columns == NULL and fun->view_columns, use it.

3. If columns == NULL and !fun->view_columns, just return.

-----------------------------------------------------------------------------

			Views in SQL

See: http://www.cs.unibo.it/~ciaccia/COURSES/RESOURCES/SQLTutorial/sqlch5.htm

5.2 VIEWS. (Virtual Tables and Data Security)

A View, as we will see, is a definition for a "virtual table" (virtual
because there is no permanent allocation of storage space) which is
assembled at reference time from selected rows and/or columns of one
or more real tables.

A view may be queried in exactly the same way as a real table.

Views are useful for two main reasons:

1) they enable users to see data, from a generalised database design, in the form most convenient for their needs.

2) they may be employed as a security mechanism for restricting user access to specific tables columns and/or rows.

The statement used to create a view has the following general form:

CREATE VIEW view-name [( column-list )]

AS SELECT column-list

FROM table-list

WHERE conditional-expression ;

You can display the specification of the views you have created using
the help statement whose general syntax is as follows:

help view view_name {,view_name}

When a view is no longer required it may be dropped from the database
with the DROP statement:

DROP VIEW table-name ;

5.2.1 Views Designed to Simplify Queries.

As we noted with Example 4.5.4, there are no direct flights from
Heathrow (HROW) to Brussels (BRUS). To simplify the query required to
list the departure times of interconnecting flights we will specify a
view called Brussels-Link.

CREATE VIEW Brussels_Link

AS SELECT DISTINCT B.FlightNo,B.FromAirport,

B.DepTime, B.ArrTime

FROM Flight A, Flight B

WHERE A.FromAirport = 'HROW'

AND A.ToAirport = B.FromAirport

AND B.ToAirport = 'BRUS' ;

Example 5.2.1 - List the FlightNo's, Airport's, Departure and Arrival
times for flights from 1500 which link Heathrow with Brussels.

SELECT FromAirport, FlightNo, DepTime, ArrTime

FROM Brussels_Link

WHERE DepTime >= '15:00';

Result:

fromai   flightn  deptime   arrtime    

BIRM     BD655    15:00     17:05      
BIRM     BD657    17:30     19:35      
BIRM     BD659    18:25     20:30