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Explain plan in Postgres has width, which (per docs) is:

Estimated average width of rows output by this plan node (in bytes).

My question: when will width be 0? Because in examples from docs, it sometimes is, like below:

EXPLAIN SELECT * FROM tenk1 WHERE unique1 < 100;

                                  QUERY PLAN
------------------------------------------------------------------------------
 Bitmap Heap Scan on tenk1  (cost=5.07..229.20 rows=101 width=244)
   Recheck Cond: (unique1 < 100)
   ->  Bitmap Index Scan on tenk1_unique1  (cost=0.00..5.04 rows=101 width=0)
         Index Cond: (unique1 < 100)
6
+150

Answer is in the article you are referring to:

Here the planner has decided to use a two-step plan: the child plan node visits an index to find the locations of rows matching the index condition, and then the upper plan node actually fetches those rows from the table itself.

Word Output here means physical read of values. Child plan does not read values, it only reads locations (addresses) of rows, then parent plan actually reads the values. And next sentence provides explanation why two step process is chosen:

Fetching rows separately is much more expensive than reading them sequentially, but because not all the pages of the table have to be visited, this is still cheaper than a sequential scan. (The reason for using two plan levels is that the upper plan node sorts the row locations identified by the index into physical order before reading them, to minimize the cost of separate fetches. The "bitmap" mentioned in the node names is the mechanism that does the sorting.)

Later in the text you'll find examples of two child processes where one process has width=244 and the other width=0. Usually child process will fetch row locations with 0 output, and then parent process actually output rows. It is not used always, but there are situations when it is faster than reading rows straight away.

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