A gap in an index can be correct when the numbers describe positions before filtering. Renumbering those same rows may be correct for a final presentation. Decide which question the number should answer before moving the indexing step.

In Power Query, Add Column, Index Column adds explicit position values. The default starts at zero and increases by one; Custom lets you specify the start and increment. The index describes the table at that transformation stage. It does not by itself establish a durable identity for the underlying record.

Put the same filter on either side of indexing

Create a small ordered practice table containing A, B and C, with a separate retained record label and an agreed sequence column. Confirm that order at the step being tested. Keep the record labels visible throughout; they are the witnesses for what each number currently refers to.

In the first query copy, add the zero-based index and then filter out B. A has index 0 and C has index 2. The gap records B's former position; it is not a missing number that must automatically be repaired.

In a second copy, filter out B before adding the same index. A now has 0 and C has 1. Both outputs contain the same two records, but C's number answers a different question: position after filtering rather than position before it.

Test what happens when the input grows

Add a new labelled row before A in the declared input sequence and repeat the comparison. Track C by its label, then inspect its index. A position can change because something earlier was inserted even though C itself has not changed.

For a real query, document the required ordering and the indexing step together. If ties or an unspecified incoming order remain, do not promise that refresh will assign the same numbers. Resolve the intended order before using an index to drive later positional transformations.

Name the column according to its role, such as IncomingPosition or DisplayPosition. If another system needs a persistent record reference, obtain an actual agreed key rather than exporting whichever small integer happens to be present today. Acceptance means the chosen stage produces the intended positions while the retained record labels still identify the correct data.

Sources: Microsoft: index columns.