check for record and proceed with index only scan

[Jayadevan M]

I am checking for existence of record in a table

cond = db.mytable(id=someid)
if not cond :
  do something

This works fine. The query executed selects all columns and adds LIMIT 1 OFFSET 0; The LIMIT and OFFSET are OK. Since I want to just check for existence of a record, how can I rewrite this so that we have only the id column selected? I am trying to avoid a table scan and have just an index scan.

[Johann Spies]

cond = db.mytable(id=someid).select(db.mytable.id)

Regards
Johann

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[Johann Spies]

Oops. That should be

cond = db(db.mytable.id==someid).select(db.mytable.id)

[Jayadevan M]

I tried that. I get the error -
<type 'exceptions.AttributeError'> 'Row' object has no attribute 'select'

By the way, the "="  works. Additional information, this function is under modules. Do I have to imprt something to make the select construct work?

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[Jayadevan M]

Sorry. It works (I got something wrong earlier) but the limit has gone away. I just have to ensure that there is ay least one record.

[Jayadevan M]

Here is how it is now ..

1) cond = db.mytable(id=someid)
generates
SELECT  <all columns> FROM mytable WHERE (id = someid) LIMIT 1 OFFSET 0;

2) cond = db(db.mytable.id==someid).select(db.mytable.id,limitby=(0, 1))
generates
SELECT  <mycolumn> FROM mytable WHERE (id = someid)  ORDER BY pk LIMIT 1 OFFSET 0;

What I would like to have is

SELECT   <mycolumn> FROM mytable WHERE (id = someid) LIMIT 1 ;

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[Anthony]

If you want to get rid of the orderby, do:

db(db.mytable.id == someid).select(db.mytable.id, limitby=(0, 1), orderby_on_limitby=False)

When you specify a limitby, by default it will orderby the PK (or add the PK to the orderby). The reason for this is to ensure a consistent ordering if you repeat the query or are using the limitby for pagination purposes (otherwise, successive queries are not guaranteed to return results in the same order).

Anthony

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