6  :  Frequently Asked Questions

Here some generic questions about the web site and about relational database technology. For questions about the databases follow the links above.
This page will be updated with new questions form users when they come in.

Web site

  • What is the difference between the "Query (stream)" and "Query (browser)" buttons?
    • The former returns data in comma-separated-values (CSV) format and in general puts no limit on the number of rows that can be returned. The "Query (browser)" button produces a result that is returned as an HTML table to the web page. It stores this result also on the server so that when the user wants to visualise the result using VOPlot or reformat the result we do not have to execute the query again. For this reason the total size of the result is limited to a maximum number of rows.
  • Why are there timeouts and why are they so short?
    • Our resources are limited and we want to ensure that queries do not run forever. It is easy to write such queries by mistake and as long as users can not cancel such queries we want to ensure that they do not have to wait overly long before the next query can be posed.
  • ...
  • Why do I get these weird "Bad gateway!" error messages?
    • Some requests produce, after some time, an error message containing text similar to the following:
    Bad Gateway! The proxy server received an invalid response from an upstream server. ...
    The likely cause of this is that instead of the database timing out, somewhere on the HTTP trail between the user machine and the database an HTTP server or proxy has decided that the user session has been idle too long and has timed out the connection. This can happen if the query sent to the database is not producing any results for the time it takes one of these servers to time out. Many servers use relatively short time outs, such as 5 minutes. I.e. even shorter than the database timeout we impose. This is one (more) reason why a batch queue for query processing would be a useful thing to have.
    It is sometimes possible to avoid this. One way is not to use "order by" for example. Such queries can sometimes only produce results once the database has retrieved all rows, which may take too long. Not though that the difference between 5 and 7 minutes is small enough that likely even without "order by" not all rows may be returned.

Relational databases

  • What is a relational database?
    • google it
  • Where can I learn more about SQL?
    • Google
  • ...