So, assuming @Vitaliy.Serdtsev and @Robert Cemper answers we have:

Index ext [type = bitmap; Extent];

Is the must for every Persistent class, with standard ID, if we want to have fast answer on "How many records"

And the easiest and fastest "How many records" in this case is:

ClassMethod HowManyRecords(ByRef recordsCount as %Integer) As %Status

{
&sql(SELECT Count(1) INTO :recordsCount FROM schema_package.table)

IF SQLCODE<0 {WRITE "SQLCODE error ",SQLCODE," ",%msg QUIT}

}

Hi Tani!

Though /_spec endpoint can provide more info, indeed we can use the embedded /api/mgmnt/ entry, which is avaialble for your REST API on:

"http://localhost:9092/api/mgmnt/v1/USER/spec/pbuttons" , if you install it in NAMESPACE USER, and run docker with:

docker run --name my-iris -d --publish 9091:51773 --publish 9092:52773 intersystemsdc/irishealth-community:2020.2.0.196.0-zpm

To visualise the API I installed the module "swagger-ui":

USER>zpm

zpm: USER>install swagger-ui

[swagger-ui] Reload START

[swagger-ui] Reload SUCCESS

[swagger-ui] Module object refreshed.

[swagger-ui] Validate START

[swagger-ui] Validate SUCCESS

[swagger-ui] Compile START

[swagger-ui] Compile SUCCESS

[swagger-ui] Activate START

[swagger-ui] Configure START

[swagger-ui] Configure SUCCESS

[swagger-ui] Activate SUCCESS

and got the nice interface to the API on: 

http://localhost:9092/swagger-ui/index.html

All set )