I am quite new to Postgres/PostGIS and have an open question regarding 'mixing' tables with different SRIDs. I know this is generally not recommended, but in my case, I will already be required to apply a union to those tables to calculate statistics over other non-geometry columns.
The only 'ST' operation I would like to apply to this 'merged' table is to calculate the area of each geometry without reprojecting the data to a common EPSG, as this might alter the covered area by some small fraction. (At least for now, that is the only operation I am interested in).
I didn't think about it too much while creating my table (and it seemed to work as expected), but now I am wondering whether or not I lucked out because the SRIDs I've tested can all be losslessly transferred to a common one.
I cannot find any resources that explain what happens when the rows have different SRIDs for the various operations. Are they all implicitly converted to a common reference system (maybe simply by selecting the first one it sees?)
If yes, would be the correct approach for my use case be to calculate the area beforehand, add it as an additional column and then merge those tables?