[Glass] Query results as Streams and indexes question

Dale Henrichs via Glass glass at lists.gemtalksystems.com
Thu Jan 5 19:07:11 PST 2017

On 1/5/17 5:38 AM, BrunoBB via Glass wrote:
> Hi All,
> For collection with indexes you can use a readStream for large query result
> (page 104 gs prg manual).
> If you try to send #readStream to a collection without an equality index
> then:
> a GsMalformedQueryExpressionError occurred (error 2710),
> reason:acceptPredicate:, Query may not be streamed.
I'd like to see the expression that generates this error ... it looks 
like you are sending #readStream to a GsQuery and not a "collection". In 
looking at the method 
GsStreamableConjunctiveClauseChecker>>acceptPredicate: where this 
particular error is signalled, there should be a more detailed 
explanation of the reason for the error ... there are 4 different 
possible reasons, so I need to see the complete error message to 
understand what may have gone wrong..
> So in the case an application need aStream on a GsQuery --> you have to pay
> NO attention to "the rule of thumb" that only a collection with more that
> 2000 objects should have indexes (page 95 gs prg manual).
The rule of thumb is "As a rule of thumb, if your collection contains 
fewer than about 2000 objects, it may not be worthwhile to create an 
index", which has nothing to do with whether or not you should use a 
stream to view results of a query.
> Is correct to say when a readStream is needed then you have to create the
> index after the first element is added ?
Whether or not you use a stream on the query results is really a 
function of how bug you think the expected result will be ... for "small 
result sets" it is not that expensive to use #queryResult.

#do: and #readStream both attempt to avoid scanning the entire result 
set and can both be used if you don't intend to to scan the entire 
result set... if you are going to touch all of the objects in the result 
set, then using #queryResult is probably a bit more efficient than using 
#do: or #readStream.
> Or my application code should be smart enough to use a readStream when there
> is an index present and a simple GsQuery without readStream when there is no
> index ?
I think you have misunderstood the reason for the error message above 
... there are certain types of queries that are not streamable (look at 
the error messages in 
GsStreamableConjunctiveClauseChecker>>acceptPredicate:for the types of 
queries that are not streamable).

And this error message has nothing to do with whether or not the 
collection has an error present or not, but with the form of the query 
that you are trying to execute... like I said earlier, if you show me an 
example of the query itself, and/or provide the full error message I can 
tell you a bit more about what may be going wrong ...


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