Hello all,

This is a generic question, but I am trying to learn enough about Dabo

to use it and I need to understand how things work before I can

proceed. I am using postgresql 9.1 and psycopg2 2.4.5. and python 2.7

on OpenSUSE 64 bit.

The only language I consider myself proficient in is dbase (not even

foxpro).

But python feels comfortable and there is a lot of 1:1 equivalency

between dbase server commands and SQL commands and it looks like Ed has

already gone down this road many years ago.

What I am unsure of is the actual technique to accomplish with

SQL/client tools what is so simple in dbase. For example:

Given a dbase table called

files

with a structure of

md5sum char 30

path char 450

file_name char 220

and indexed (or sorted) in the order of

md5sum + path + file_name

run the following script

<CODE>

********************** mark_del.prg **********************

*mark all records with identical md5sums as deleted

*except for last one

*initialize some variables

store "" to m_md5sum

store 0 to m_hit_cntr

store 0 to m_place_holder

*open the table for reading and writing

use files

*begin to process the table

*iterate until end of table

do while .not. eof()

*save the md5sum for comparison

store md5sum to m_md5sum

*save position in table if needed to come back here

*recno() is probably like oid in postgresql (unless one has defined

one's own sequence)

store recno() to m_place_holder

*initialize the counter for duplicate md5sums

store 0 to m_hit_cntr

*look for instances of duplication by counting identical md5sums

*iterate until a change in md5sum

do while md5sum = m_md5sum .and. .not. eof()

*increment the counter

store m_hit_cntr + 1 to m_hit_cntr

*scroll one record

skip

enddo

*Are there duplicates?

if m_hit_cntr > 1

*go back to starting point

goto m_place_holder

*decrement the counter by the one record to leave unmarked

store m_hit_cntr -1 to m_hit_cntr

*go down the list again and mark records as deleted

*iterate until counter = 0

do while m_hit_cntr > 0 .and. .not. eof()

*mark record as deleted

delete

*decrement the counter

store m_hit_cntr -1 to m_hit_cntr

*scroll one record

skip

enddo

*scroll past the record to retain

skip

endif

*scroll to next record to begin the sequence again

skip

enddo

*close the table

use

*all done now

</CODE>

The code above is untested, but those of you with dbase experience

should get the idea.

This is a real task by the way (I'm cleaning up my hard disk). There

are 1,100,000 records in the table about 47% of them duplicates.

My SQL table is identical to the table above with the addition of two

more fields:

del boolian

recno long long int (sequence unique)

I can mimic the above operations in postgresql/python except for one thing.

I can connect to the database and table

I can iterate through the table

I can scroll up and down the table

I can update the field del in the table

Because of the size of the table, I think a named cursor is appropriate.

The problem I am having is that each update requires a commit statement

and a commit statement closes the named cursor. So one loses his place.

I understand that psycopg2 named cursors now have an attribute

withhold

and if that is set to true, then a commit will not close the named

cursor (unless a rollback is done), but I have not been able to make

this work yet.

While studying this, I thought of an approach to do what I am trying to

do. This approach involves the use of two cursors that I call a

"walking" cursor and a "working" cursor.

This seems so messy to me so I wanted to see how Dabo did it.

I have been reading the Dabo code and I notice the use of what is called an

AuxCursor

Which is described at

line 439 of dCursorMixin.py

Where it says:

...because it won't harm the contents of the main cursor....

So (finally) my questions are:

Is the use of an AuxCursor the only way to do what I want to do (unless

I can get the withhold attribute to behave)?

If there is another way, where is the best description of it so I can

read about it?

Thanks in advance,

Bill House

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