[FIX] OWSql: enforce data download for non PostgreSQL databases

[robertcv]

Issue

Fixes #3043

Description of changes

SqlTable has poor support for non-PostgreSQL databases. This enforces the download of the selected table so that it works as a normal Table.

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #3178 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3178      +/-   ##
==========================================
+ Coverage   82.54%   82.54%   +<.01%     
==========================================
  Files         337      337              
  Lines       58431    58446      +15     
==========================================
+ Hits        48229    48246      +17     
+ Misses      10202    10200       -2

[Ryujjin]

Thank you for the patch but with the auto download always enabled the data is to large to fit in memory. No data is loaded and thus no widgets can be used to manipulate the information. What is the workaround for working with large databases?

Thanks!

[ajdapretnar]

I thought we went with sampling here... But it could be we discarded this option for some reason.

@Ryujjin Did you try using Data Sampler immediately after SQL and sample it yourself?

[Ryujjin]

Thanks for the reply, I need to be able to pick date ranges or time intervals, last I looked the sampler appeared to be a random range but I could be wrong will look at it again.

Thanks!

[lanzagar]

If you know what kind of subset of a large data set you want to explore it will probably be best to use an SQL select query to get just that data. This will also be much faster than transferring large amounts of data and sampling on the client.