REINDEX and blocking SELECT queries

Hi all,

According to the manual, running REINDEX does not take any locks
on the parent table which block read operations.

> REINDEX locks out writes but not reads of the index's parent table.

REINDEX
https://www.postgresql.org/docs/9.5/static/sql-reindex.html

However, I heard some complains from a few people that this explanation
would lead some misunderstanding that REINDEX would not block
SELECT queries on the table.

Actually, REINDEX blocks SELECT queries, maybe in the planning phase.

This explanation is very critical for many people because they are planning
to run REINDEX online with believing in the explanation.

So, I think we should improve this explanation precisely not to lead
such misunderstanding.

What do you think? Any comments?

Regards,
--
Satoshi Nagayasu <snaga@uptime.jp>