Single table of all chat conversations or separate tables?
If this is an actual service that you would like to create and not just an academic exercise, then it would be best to start with some realistic expectations and grow from there. Many years ago I implemented something like this at the day job where employees could communicate with one another in a Microsoft Teams-like fashion (though before Teams was a thing). The system is still being used today and sees anywhere between 5 and 25 messages per minute between the hours of 7:30am and 2:45am local time. It is used by about 18,500 employees scattered across the globe.
Current statistics (as of 00:00:00 UTC today):




Item
Number
Note




Posts
35,641,804
As per HR and Legal requirements, no records are deleted, but they are instead &quot;soft deleted&quot;.


Files
2,613,017
Same as above


Channels
74,331
 


Accounts
24,459
6,000+ idle/expired accounts




Each one of these items has its own table, and there are additional tables that enable additional functionality, such as ChannelMember, which specifies which Accounts are members of a Channel and what level of permission they have (Read-Only / Read-Write).
Posts are saved in Markdown format and have no realistic size limit. People are free to use the space as they see fit and, so long as nobody reports an Account or Channel as being inappropriate, no communications are monitored or shared with management.
Search is handled by splitting posts apart on a word-by-word basis and storing unique words in a PostSearch table, which is then used for somewhat faster lookups. Because the words are split apart, search results can be &quot;weighed&quot; for their relevance before being returned to the requester.
This runs on a MySQL database with a PHP-based API handling all communication between the front-end applications and the database.
Now the fun stuff:




Action
Average Response Time




Loading Channels a Person Belongs To
0.18 seconds


Loading a Timeline view of recent Posts
0.21 seconds


Loading the most recent 250 Posts in a Channel
0.13 seconds


Publishing a new Post
0.51 seconds


Searching for Posts with 3 terms
1.33 seconds


Searching for Posts with 5+ terms
1.61 seconds




These are API response times, meaning requests are first authenticated, processed, then returned in a JSON format.
The database server is running MySQL 8.0.23 and Ubuntu 20.04 LTS on a db.m6g.xlarge instance on AWS. There are three web servers, all running Apache and Ubuntu 20.04 LTS on t4g.medium instances.
Unless you have tens of millions for marketing a hot new service that will supplant the existing big players, you may not need to over-think the structure just yet. Build something specific and see whether it catches on or not. If it does, then you can look at getting it to scale.