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<div class="moz-text-flowed" style="font-family: -moz-fixed;
font-size: 12px;" lang="x-unicode">Dear DAL and Apps members,
<br>
<br>
Since I do not attend to this interop, I would like to highlight
quickly one of my last development concerning TAP because I think
it may be interested to either do the same in your own TAP service
or merely use it. As suggested by the title of this email it is
about adding metadata in TAP.
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<br>
(I send this email also to Apps because of the last point I make
in this email: a compatibility with a new feature of TOPCAT)
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** Columns metadata
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The idea is to add basic statistics like a count, min, max, ...
for some numerical columns of tables published in a TAP service.
For that, I have just added the following columns in
TAP_SCHEMA.columns:
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- min_value
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- max_value
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- mean
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- std_dev
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- q1 (i.e. first quartile)
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- median (i.e. second quartile)
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- q3 (i.e. third quartile)
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- filling (number of rows having a NOT NULL value for this
column)
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Except for "filling" which must be an integer (INTEGER or BIGINT
in PostgreSQL), I have chosen to set all these columns as DOUBLE
PRECISION since most of the columns to describe are, in the
"worst" case, double values.
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When no statistics can be provided for a column, all these
additional metadata would be NULL.
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** Tables metadata
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In addition, I have also added another column in
TAP_SCHEMA.tables:
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- row_count (of type INTEGER or BIGINT)
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** VOSI description of tables
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Since in TAP all tables and columns metadata MUST be the same in
TAP_SCHEMA and /tables, I have also updated our /tables resource.
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Besides, on a recommendation of Mark Taylor, I designed and linked
a simple XSD schema in order to have a valid XML document. You
can find this schema at the following address:
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<br>
<a class="moz-txt-link-freetext"
href="http://gaia.ari.uni-heidelberg.de/tap-stats.xsd">http://gaia.ari.uni-heidelberg.de/tap-stats.xsd</a>
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** Visibility in TOPCAT
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Thanks to Mark Taylor, any custom metadata (non-standard TAP
columns) can be displayed in the last version of TOPCAT. Thus, all
the statistics described above can be seen there for our Gaia TAP
service (n.b. you can find this TAP service easily in the registry
with the keywords "Gaia" and "ARI", but in case you can not, here
is the root access URL: <a class="moz-txt-link-freetext"
href="http://gaia.ari.uni-heidelberg.de/tap">http://gaia.ari.uni-heidelberg.de/tap</a>).
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** Last words...
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According to me all these basic statistics may be useful to
discover the content of a table, especially when this one is as
large as Gaia, PPMXL, 2MASS, ... It can indeed prevent some users
to perform apparently simple and short queries such as "SELECT
COUNT(*) FROM a_big_table" which on the contrary to what most
people think is not often a quick query on large tables. Having
already computed such information is then an economy of time and
resources for the users and the server.
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Finally, I am not trying to convince anybody to have such
metadata, but I just want to highlight a possible extension of TAP
helping in simple data discovery. Besides, this use-case also
demonstrates how easy it could be to add more simple metadata
inside a TAP service. So maybe it could be interested, if
possible, to write an appendix about that in the next version of
TAP or just as an IVOA note. What do you think?
<br>
<br>
If anybody has questions or wants further details about the TAP
"extension" I presented here, do not hesitate to ask ; I am not at
the interop, but I am fully available by email <span
class="moz-smiley-s1" title=":)"></span>
<br>
<br>
Regards,
<br>
Grégory
<br>
<br>
<br>
PS: For those who are interested, I also provide histograms and
sky-maps (using Healpix) for most of the published columns on the
page <a class="moz-txt-link-freetext"
href="http://gaia.ari.uni-heidelberg.de/tap/tables">http://gaia.ari.uni-heidelberg.de/tap/tables</a>.
Both can be downloaded as images but also as tables that you can
then display/process as you want (e.g. display the histogram in
TOPCAT, display and navigate inside the Healpix map in Aladin,
...).
<br>
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