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A.9.8 Table Access Protocol (TAP) Query

Table Access Protocol Window

Table Access Protocol Window

The Table Access Protocol (TAP) load window can be opened using the TAP Query button () in the Control Window's toolbar or VO menu, or the Load Window's toolbar. It allows you to use the TAP protocol to make freeform queries of remote database services using an SQL-like language. This is a powerful capability, giving you full access to remote data holdings; it's similar to systems like SDSS's CasJobs, but using the IVOA-endorsed TAP standard means that you can use the same interface for many different remote datasets.

Using TAP is one of the more complicated things you can do in TOPCAT, but it's well worth getting to grips with because of the capabilities it gives you for customised access to all kinds of remote data.

In order to interrogate the remote database you will have to write queries in Astronomical Data Query Language (ADQL). ADQL is essentially a particular dialect of SQL, so if you have used SQL before, you should be able to write ADQL queries. A tutorial on writing ADQL/SQL is beyond the scope of this manual, but the Use Service tab provides an Examples menu which will give you a good start in using it. You can find some introductions and tutorials elsewhere, for instance at http://docs.g-vo.org/adql (Markus Demleitner) or http://is.gd/ADQLTutorial (Simon Murphy). Some TAP services also provide tutorials or examples on their web pages.

Once your query is written, you can submit it in either synchronous or asynchronous mode. Synchronous queries begin to execute right away and should have a relatively short runtime; asynchronous queries are submitted to a server, run for a potentially long time, and retrieve the result later.

The window is composed of four tabs, of which you can see one at a time (some of these tabs themselves contain tabbed parts). You will use two or three of them when submitting a TAP query. Each tab is described in more detail in the following subsections, but an overview of their operation is like this:

Select Service tab
Chooses the TAP service you wish to query, either using a registry search or by entering the service URL directly. Once you have chosen a service you can move to the Use Service tab.
Use Service tab
Displays information about the service and the tables it holds, and allows you to enter the text of the query and submit the job. If the job is submitted synchronously, loading will begin directly, but in the asynchronous case, you will move to the Running Jobs tab.
Running Jobs tab
Shows a list of the asynchronous query jobs you have submitted, with details on their progress.
Resume Job tab
Optionally allows you to resume an asynchronous query started in some earlier session.

When you first visit this window, the Select Service tab will be visible, and when an asynchronous query has been submitted the Running Jobs tab will be visible. If you want to submit another query to the same service, or use a different service, just select Use Service or Select Service respectively, by clicking on the appropriate tab at the top.

This window offers some menus additional to the standard VO window menus:

TAP menu
This contains some configuration options specific to Table Access Protocol service interaction. It has the following sub-items:
Reload ()
Reloads information displayed in the window from the server. This may be updating job status information or reloading table or service metadata; the exact behaviour depends on which tab is currently visible.
Job Deletion submenu
Allows you to choose when asynchronously submitted jobs will by default be deleted from the server. Once they are deleted they will no longer be visible in the Running Jobs tab. If they are not deleted in this way, they will stay on the server until they are deleted according to the server's deletion policy. Choose one of the available options:
  • On Completion: deleted when it has completed with either success or error status
  • On Exit: deleted when TOPCAT shuts down
  • Never: not deleted (except according to server policy)
You can override these settings once a job has started by using the controls at the bottom of the Running Jobs tab. The settings in this menu affect the default value of the Delete On Exit checkbox. If you want to resume the job in a later session, you'll have to make sure that the job is not deleted on completion or exit.
Metadata Acquisition submenu
Controls how the application loads service metadata - information about the tables and columns provided by the service. The following options are available:
  • Auto: make a sensible decision; this generally means TAP_SCHEMA_C for services with very many tables, and TableSet otherwise.
  • TableSet: reads the document at the /tables endpoint of the TAP service; all table metadata is loaded up front.
  • TAP_SCHEMA_C: reads metadata from TAP_SCHEMA tables; all schemas, tables and foreign keys are read at once, columns are read only when required.
  • TAP_SCHEMA_CF: reads metadata from TAP_SCHEMA tables; all schemas and tables are read at once, columns and foreign keys are read only when required.
  • TAP_SCHEMA: reads all metadata at once from the TAP_SCHEMA tables.
  • VizieR: uses the 2-stage TableSet from /tables as implemented (at time of writing) by the TAPVizieR service; columns are read only when required (experimental, may be withdrawn).
  • CADC: uses the 2-stage TableSet from /tables as implemented (at time of writing) by CADC services; columns are read only when required (experimental, may be withdrawn).
  • VOSI11-1step Uses VOSI 1.1-style detail-sensitive requests to the /tables endpoint. detail=max is specified initially to indicate that full metadata up-front metadata load is preferred.
  • VOSI11-2step Uses VOSI 1.1-style detail-sensitive requests to the /tables endpoint. detail=min is specified initially to indicate that on-demand column load is preferred.
  • VOSI11 Uses VOSI 1.1-style detail-sensitive requests to the /tables endpoint. No detail specification is made initially, so the service decides whether up-front loading is done.
You will usually want to stick with the default value of Auto. However, if you're on a slow link, and it's taking a long time to display any metadata for medium-sized services, you might want to switch to TAP_SCHEMA_C/CF, or VizieR for talking to TAPVizier.
Response Format submenu
Configures how the application requests that VOTable results are to be returned. The options (TABLEDATA, BINARY, BINARY2) correspond to the different defined VOTable serialization formats. The selected format is only actually requested if the service has indicated support for it, otherwise the service default is used.
Upload Format submenu
Configures how tables are transmitted to the service for upload if an upload query (one involving a TAP_UPLOAD.* table) is performed. The options (TABLEDATA, BINARY, BINARY2) correspond to the different defined VOTable serialization formats. The choice shouldn't affect any results, but it may affect required bandwidth and some services may (though shouldn't) have non-standard requirements for serialization format.
Service Discovery
Configures how TOPCAT searches for available TAP services. There are currently two options, GloTS, which uses the Global TAP Schema service list maintained by GAVO, and Reg Prototype which uses a proposed convention based on the IVOA Registry. You are advised to use the default value (currently GloTS) unless you understand what you're doing here.
HTTP gzip
If checked, this option sends Accept-Encoding: gzip headers with most of the HTTP requests when communicating with the TAP service. That ought to reduce required bandwidth considerably, but only if the service chooses to honour this request. The setting should have no effect on the actual results, but may affect performance.
Edit menu
Provides options concerned with editing ADQL text. This is only active when the Use Service tab is visible, and it is described in that section.

Note: TAP is a complex protocol, and at time of writing, many TAP services still do not provide full and error-free implementations. Unexpected behaviour may be a result of service implementation deficiencies. Hopefully that will improve over time.


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Next: Select Service tab
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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk