Here’s yet another entry in the list of useful things I learned from Ehren von Lehe on the Power Query MSDN forum: Table.Buffer() does not buffer nested table, record or list values inside the cells of a table. From this thread:
Buffering is shallow. It will force the evaluation of any scalar cell values, but will leave non-scalar values (records, lists, tables, etc.) as-is.
It’s actually really difficult to come up with a simple demo query to prove this though (the Power Query engine is too clever about not evaluating things it doesn’t need for the final output of a query), but it’s fairly easy to understand the principle. Whenever you have an expression that returns a table something like this:
…if you use Table.Buffer() on this table it will only buffer the scalar values (in this case the text values in the Name and Signature columns). The nested table values, as in the highlighted cell, will not be buffered in memory and if you try to access the contents it may result in another call back to the underlying data source.