I was thinking of adding another column called "Test / Train" in which a 0 would indicate test and a 1 would indicate train data, and then as I'm collecting data end the train data collection phase and start entering each row with 1 in that column. Then using select rows widget to select all rows with "Test / Train" = 0 or 1. However, under PCA, the original columns go away so this wouldn't work!
How then, can I just select all rows with row id > k? I need this because the past data is supposed to predict a smaller set of future data, so I want to be sure that it works and not just select the test set randomly from the training data (which is an option in Test & Score).