USER>d ##class(mp.test).Fill(5000000)

USER>d ##class(mp.test).Query()
count=3833346

first 5 [1:5]
ID prop3
3 3
4 3
24 3
30 3
97 3

5 Rows(s) Affected
.000328 secs

last 5 [3833342:3833346]
ID prop3
4999798 1
4999817 1
4999836 1
4999866 1
4999947 1

5 Rows(s) Affected
2.884304 secs

PS: for those who put a minus. May I ask why?

Have you tried this?

There is at least one case where this encoding style is acceptable - it is a generator method. Just look at the code generated by &sql() or CSP, for example.
No wonder there are two versions for javascript libraries: for the developer and for production (processed by the minimizer).

And what if the answer is given, but the author either does not want to accept the answer at all or does not make a more elegant decision?

https://community.intersystems.com/post/there-way-defermine-column-jsono...
https://community.intersystems.com/post/creating-json-objects-objectscri...

My EAV implementation is the same as your Approach 3, so it will work fine even with fully filled 4.000.000 attributes.
Since the string has a limit of 3,641,144, approaches with serial and %List are dropped.

All other things being equal, everything depends on the specific technical task: speed, support for Objects/SQL, the ability to name each attribute, the number of attributes, and so on.

Entity–attribute–value model is usually used for this purpose.

I have already written about this at the time: SQL index for array property elements.

Here is a ready-made example (works even in last Caché):

Class dc.test Extends %Persistent
{

Property title As %VarString;

Property author As %VarString;

/// do ##class(dc.test).test()
ClassMethod test()
{
  &sql(truncate table dc.test)
  
  &sql(insert into dc.test(title,author)
    select 'For Whom the Bell Tolls','Hemmingway' union
    select 'The Invisible Man','Ellison' union
    select 'Tender is the Night','Fitzgerald')
  
  set provider=##class(%ZEN.Auxiliary.altJSONSQLProvider).%New(),
      provider.sql="select title,author from dc.test",
      provider.arrayName="items",
      provider.maxRows = 0,
      provider.%Format = "tw"
  
  do provider.%WriteJSONStreamFromSQL(.stream,,,,,provider)
  
  set json={}.%FromJSON(stream),
      json.results=json.items.%Size()

  write json.%ToJSON()
}

}

Result:

USER>do ##class(dc.test).test()
{"items":[{"title":"For Whom the Bell Tolls","author":"Hemmingway"},{"title":"The Invisible Man","author":"Ellison"},{"title":"Tender is the Night","author":"Fitzgerald"}],"results":3}

Google the JDBC Driver for Excel.
Here is a good option: HXTT Excel Pure Java JDBC (1.2, 2.0, 3.0, 4.0, 4.1, 4.2, 4.3) Drivers for MS Excel (XLS, XLSX)

Class dc.test [ Abstract ]
{

ClassMethod ValType(ByRef var) As %String
{
  s a(-1)="undefined",a(0)="empty",a(1)="string",a(2)="integer",a(3)="float",a(4)="double",a(5)="object",a(6)="list",a(7)="array",a(8)="bitstring"
  s r=$s('$d(var):-1, $isobject(var):5, $d(var)>9:7, var="":0, $lv(var):6, $l(var)>254:1, 1:$tr($a($lb(var),2),1245678,1122334))
  i r=1 {
    s s=##class(%Utility).FormatString(var)
    s:($f(s,"$c(")=4)||($f(s,"$zwc(")=6) r=8
  }
  q a(r)
}

/// d ##class(dc.test).test()
ClassMethod test()
{
  s old=$system.Process.Undefined(2)
  try{

    w ##class(%Utility).FormatString(undef)," => ",..ValType(.undef),!

    f i="","1",1,1.1,$double(1.1),##class(%ListOfDataTypes).%New(),$lb(),$factor(1),$c(7) {
      w ##class(%Utility).FormatString(i)," => ",..ValType(.i),!
    }
    
    s (i,i(1))=""
    w ##class(%Utility).FormatString(i)," => ",..ValType(.i),!

  }catch{
  }
  
  d $system.Process.Undefined(old)
}

}

Result:

USER>d ##class(dc.test).test()
"" => undefined
"" => empty
1 => string
1 => integer
1.1 => float
$double(1.1000000000000000888) => double
"1@%Library.ListOfDataTypes" => object
$lb() => list
$zwc(128,4)_$c(1,0,0,0)/*$bit(1)*/ => bitstring
$c(7) => bitstring
"" => array

InterSystems IRIS Community Edition Limitations
Also see %SYSTEM.License, e.g. method MaxConnections

It can be simpler:

Property HowManyRecords As %Integer [ Calculated, ReadOnly, Required, SqlComputeCode = {n r,SQLCODE &sql(select count(*) into :r from schema_package.table) s {*}=$s(SQLCODE:-1,1:r)}, SqlComputed ];

Are a couple of points:

1. The macro "ClassSQLTable" is defined in the method %Library.EnsembleMgr:UpgradeUTCIndices
2. I would prefer to use ##class(%DeepSee.Utils).%GetSQLTableName(pClass As %String, pVerifyExists=0)

Unfortunately, there are errors in the methods GenExportBinaryStream/GenImportBinaryStream in the class %JSON.Generator.

Steven, are you sure that wanted to answer namely me?