I have some summary-indexed data over the last couple of months. I was wondering if its possible to add another field to this data. Is it possible to modify the underlying query to add this new field and get past data "fixed"?
For example, if I have the following query: index=blah type=a | sitimechart field1
Could I modify this to be: index=blah type=a | sitimechart field1 by field2
No it is not possible. Like all Splunk data, you basically can't modify it once it's been indexed.
You can however delete the old data and use the backfill script to re-generate the new data.
This is equivalent to what you're asking for anyway. There is no advantage to having the old summary data in your example. Your new summary would have to be regenerated from the original data, as it is impossible to construct (or "modify") "sitimechart field1 by field2" from "sitimechart field1" without simply regenerating from original.