This PR is V2 of the[ PR created by @psarna.](https://github.com/scylladb/scylladb/pull/11560).
I have:
- copied the content.
- applied the suggestions left by @nyh.
- made minor improvements, such as replacing "Scylla" with "ScyllaDB", fixing punctuation, and fixing the RST syntax.
Fixes https://github.com/scylladb/scylladb/issues/11378Closes#11984
* github.com:scylladb/scylladb:
doc: label user-defined functions as Experimental
doc: restore the note for the Count function (removed by mistatke)
doc: document user defined functions (UDFs)
(cherry picked from commit 7cbb0b98bb)
Fix https://github.com/scylladb/scylladb/issues/11373
- Updated the information on the "Counting all rows in a table is slow" page.
- Added COUNT to the list of selectors of the SELECT statement (somehow it was missing).
- Added the note to the description of the COUNT() function with a link to the KB page for troubleshooting if necessary. This will allow the users to easily find the KB page.
Closes#11417
* github.com:scylladb/scylladb:
doc: add a comment to remove the note in version 5.1
doc: update the information on the Countng all rows page and add the recommendation to upgrade ScyllaDB
doc: add a note to the description of COUNT with a reference to the KB article
doc: add COUNT to the list of acceptable selectors of the SELECT statement
(cherry picked from commit 22bb35e2cb)
