Proposal to Enable/Disable Index using ALTER INDEX

Hello hackers,

This is my first time posting here, and I’d like to propose a new feature
related to PostgreSQL indexes. If this idea resonates, I’d be happy to
follow up with a patch as well.

*Problem*:
Adding and removing indexes is a common operation in PostgreSQL. On larger
databases, however, these operations can be resource-intensive. When
evaluating the performance impact of one or more indexes, dropping them
might not be ideal since as a user you may want a quicker way to test their
effects without fully committing to removing & adding them back again.
Which can be a time taking operation on larger tables.

*Proposal*:
I propose adding an ALTER INDEX command that allows for enabling or
disabling an index globally. This could look something like:

ALTER INDEX index_name ENABLE;
ALTER INDEX index_name DISABLE;

A disabled index would still receive updates and enforce constraints as
usual but would not be used for queries. This allows users to assess
whether an index impacts query performance before deciding to drop it
entirely.

*Implementation*:
To keep this simple, I suggest toggling the indisvalid flag in pg_index
during the enable/disable operation.

*Additional Considerations*:
- Keeping the index up-to-date while it’s disabled seems preferable, as it
avoids the need to rebuild the index if it’s re-enabled later. The
alternative would be dropping and rebuilding the index upon re-enabling,
which I believe would introduce additional overhead in terms of application
logic & complexity.
- I am also proposing to reuse the existing indisvalid flag to avoid adding
new state and the maintenance that comes with it, but I’m open to feedback
if this approach has potential downsides.
- To keep the scope minimal for now, I propose that we only allow enabling
and disabling indexes globally, and not locally, by supporting it
exclusively in ALTER INDEX. I would love to know if this would break any
SQL grammar promises that I might be unaware of.

I would love to learn if this sounds like a good idea and how it can be
improved further. Accordingly, as a next step I would be very happy to
propose a patch as well.

Best regards,
Shayon Mukherjee