What does TOP mean ?

[Douglas Tody]

TOP in SSAP is similar to what you get in a Google search, where some
automatic, smart heuristic is used to order the (potentially very many)
records that are found, returing the TOP=N records to the client.
Without this capability a large search with many hits would be
worthless.  TOP in SQL/ADQL (if implemented) returns the first N records
of whatever query the client requested.  The ordering is likely not well
defined, unless you add an ORDER BY clause or some such.

Unfortunately few actual SSAP services implement the protocol correctly,
and TOP is an advanced capability since heuristics like this are
inherently hard to implement (my NVO/VAO SSA reference implementation
does prototype TOP).  Clearly smart ordering of the query response would
be a useful feature in an advanced query, regardless of whether the
client uses the TOP constraint.  Think of it for example as a
multi-dimensional distance of each record from the central point in the
N-space requested by the client.

 	- Doug


On Sat, 25 Apr 2015, Petr Skoda wrote:

>
> Dear all
>
> during teaching VO at several universities last several weeks I have got and 
> interesting question to which I cannot find answer ....
>
> What does mean TOP in ADQL mean and how the TOP is implemented in various 
> SSAP services ?
>
>
> In ADQL (ver 2.0) is said only that TOP returns n records
>
> while In SSAP 1.1 is written in 4.1.2.15 that TOP returns n TOP RANKING 
> records using some score function ....
>
> "
> Metrics such as distance from the specified position, or the degree of 
> overlap with a specified bandpass or time interval, determine the score.
> "
>
> just trying randomly ADQL over TAP in Topcat to SDSS8 in Eddinburg it seems 
> that TOP  on PhotoPrimaryAll where psfMag_g < 6 tries to oreder by the psfMag 
> and then limit ....
>
> But it may be just my subjective feeling .....
>
>
> And does anybody implemented TOP in scoring manner in SSAP ?
>
> Best regrads,
>
> Petr
>
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> *  Stellar Department                         +420-323-620361           *
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