Can thumbnail size in db be changed after db creation

[ubacher]

I have my thumbnail size at 400 pixel. I did a calculation what reducing it to 300 would save. It would decrease my big db by several GB.
I changed it now to 300.
Question: will this affect only future files added?
and
Is there a (practical) way to reduce the thumbnail size for existing files?

[Mario]

Question: will this affect only future files added?

Yes.

Is there a (practical) way to reduce the thumbnail size for existing files?

By forcing a rescan using Shift+Ctrl+F5, which will also create a new thumbnail based on the new thumbnail size settings.

How many million files to you plan to manage in IMatch?

Saving maybe 30-35% for thumbnails, is this really worth the effort?
The average storage size for a 300 pixel thumbnail is between 16 and 18KB.
IMatch stores thumbnails as JPEG images, unless they have alpha channels, then as PNG files. Both compress quite well.

A database with a million managed assets and a thumbnail size of 300 pixels needs between, say, 25 to maybe 35 GB, depending on the amount of metadata etc.
The size of the database on disk is irrelevant for IMatch performance. A SSD with 512 GB of storage costs only about 60-70 US$ these days.

You can save a lot more disk space by reducing the number of images you keep in the IMatch cache (assuming you are not only using JPG files in IMatch). See The Cache for information on how to control cache size and purging cache images.

[ubacher]

My db has 23.7 Gb. (370k files). I do not use cache images.
The calculated savings from 400 to 300 pixels would be about 5 Gb.

MY P&G file is 13.3 GB and I assume the saving there would be about the same. (or do the thumbnails also get
compressed significantly?).

My attitude is that every little bit helps to speed up the system - if the effort to do so is tolerable.

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A question that came to mind ( Since I don't use the cache): I assume that using cache images just speeds up viewing those cached images. Is there a hidden speed penalty for other operations?

[Mario]

A question that came to mind ( Since I don't use the cache): I assume that using cache images just speeds up viewing those cached images.

Definitely. Depending on the RAW format you use, WIC can load a RAW in between 2 and 10 seconds. IMatch can load a cache image in between 0.1 and 0.5 seconds.

MY P&G file is 13.3 GB

Pack & Go is a tool to comfortably migrate/copy IMatch settings and databases between multiple computers. For users who use e.g. a desktop in the studio and a notebook on the road.

It is not a backup tool!
I've only made it so after learning that so many users never do backups. And then Pack & Go is better than nothing...

Modern "image" backup tools like Macrium Reflect or TrueImage  backup modified sectors when a file changes.
If you have a database with 25 GB and work with it for a day, and then create an incremental backup with one of these specialized backup applications, the backup volume on disk is between a few dozen to a few hundred MB. Not GB.