REINDEX vs VACUUM

+ pgsql-admin
Would appreciate any insights. Thanks

On Sat, Dec 31, 2022 at 1:04 PM Hao Zhang <kennthhz@gmail.com> wrote:

Here is my understanding:

REINDEX recreates the index from scratch, using the data stored in the
underlying table. It is the same as dropping and recreating the index
manually, with regard to the impact on the index file. It can free up
physical space in the file system. REINDEX will not vacuum the index.

VACUUM does many different things. One of them is vacuuming indexes (for
the underlying table that is being vacuumed). VACUUM will remove index
entries that are pointing to dead rows in the underlying table. VACUUM
will not rebuild the entire index.

VACUUM recycles free index blocks (using FSM), but it does not (usually)
free up space for the file system physically. VACUUM FULL does that.

VACUUM FULL will vacuum the index, but it will do it by making a copy of
the index (file) and reorganizing its content in order to free up space
physically. In that regard, it is like REINDEX. Both VACUUM FULL and
REINDEX will block reads from the index during the process (by taking an
ACCESS EXCLUSIVE lock).

Regards,

tamas

2022. 12. 31. 22:04 keltezéssel, Hao Zhang írta:

I don't think VACUUM FULL (copy the table, create new indices and other
metadata all in one command) actually vacuums tables.  It's a misleading name.

Something like REBUILD TABLE would be a better name.

On 1/4/23 07:25, Rébeli-Szabó Tamás wrote:

Here is my understanding:

REINDEX recreates the index from scratch, using the data stored in the
underlying table. It is the same as dropping and recreating the index
manually, with regard to the impact on the index file. It can free up
physical space in the file system. REINDEX will not vacuum the index.

VACUUM does many different things. One of them is vacuuming indexes (for
the underlying table that is being vacuumed). VACUUM will remove index
entries that are pointing to dead rows in the underlying table. VACUUM
will not rebuild the entire index.

VACUUM recycles free index blocks (using FSM), but it does not (usually)
free up space for the file system physically. VACUUM FULL does that.

VACUUM FULL will vacuum the index, but it will do it by making a copy of
the index (file) and reorganizing its content in order to free up space
physically. In that regard, it is like REINDEX. Both VACUUM FULL and
REINDEX will block reads from the index during the process (by taking an
ACCESS EXCLUSIVE lock).

Regards,

tamas

2022. 12. 31. 22:04 keltezéssel, Hao Zhang írta:

What is the difference between reindex and vacuum's impact on index file?
I deleted an indexed row. Running either vacuum or reindex shows the
index entry for the row is removed from the index page. I was under the
impression that only reindex will remove dangling index entries. I am
guessing that vacuum will not shrink the index file and will only add
deleted index entries in the free space file for the index? But REINDEX
is recreating the index file from scratch so it is like vacuum full for
index?

Thanks

--
Born in Arizona, moved to Babylonia.

I have looked more into it and have found that VACUUM FULL (and CLUSTER)
does in fact rebuild indexes, see for example:
https://github.com/postgres/postgres/blob/c8e1ba736b2b9e8c98d37a5b77c4ed31baf94147/src/backend/commands/cluster.c#L1463

I have also run some tests and have come to understand that REINDEX
seems to "vacuum" the index in the sense that a subsequent VACUUM on the
underlying table will not find any removable entries in the index.

Ron is right, the term "vacuum" may be misleading.

2023. 01. 04. 16:34 keltezéssel, Ron írta:

On 1/4/2023 9:34 AM, Ron wrote:

I don't think VACUUM FULL (copy the table, create new indices and
other metadata all in one command) actually vacuums tables.  It's a
misleading name.

Something like REBUILD TABLE would be a better name.

Well s***.

That explains a lot. Thanks for clearing that up for me.

On 2023-01-04 09:34:42 -0600, Ron wrote:

I don't think VACUUM FULL (copy the table, create new indices and other
metadata all in one command) actually vacuums tables.  It's a misleading
name.

Move all the stuff from the living room to the bedroom and then jettison
the living room.

Isn't that how you normally vacuum your living room?

hp

--
_ | Peter J. Holzer | Story must make more sense than reality.
|_|_) | |
| | | hjp@hjp.at | -- Charles Stross, "Creative writing
__/ | http://www.hjp.at/ | challenge!"

Von: "Peter J. Holzer" <hjp-pgsql@hjp.at>
On 2023-01-04 09:34:42 -0600, Ron wrote:

I don't think VACUUM FULL (copy the table, create new indices and other
metadata all in one command) actually vacuums tables.  It's a misleading
name.

Move all the stuff from the living room to the bedroom and then jettison
the living room.

Isn't that how you normally vacuum your living room?

Well, yeah, I wouldn't expect the table to be *copied*. After all, there's
not that much use for more than one per living room.

Karsten

On 2023-01-05 12:34:08 +0100, Karsten Hilbert wrote:

Von: "Peter J. Holzer" <hjp-pgsql@hjp.at>
On 2023-01-04 09:34:42 -0600, Ron wrote:

I don't think VACUUM FULL (copy the table, create new indices and other
metadata all in one command) actually vacuums tables.  It's a misleading
name.

Move all the stuff from the living room to the bedroom and then jettison
the living room.

Isn't that how you normally vacuum your living room?

Well, yeah, I wouldn't expect the table to be *copied*. After all, there's
not that much use for more than one per living room.

Well, you won't have the old table anymore if you leave it in the living
room.

On a more serious note: It it pretty much impossible to move anything
inside a computer. To move something you have to

* copy it and then delete (overwrite) the original, or
* leave it where it is and refer to it from a different location (which
typically means copying and deleting the reference)

hp

--
_ | Peter J. Holzer | Story must make more sense than reality.
|_|_) | |
| | | hjp@hjp.at | -- Charles Stross, "Creative writing
__/ | http://www.hjp.at/ | challenge!"