Recursive CTE have performace issue, need suggestion to optimize query

I wanted to get TOP 5 records from log table where “Approve date” is changed like NULL to value and vice versa. Date value is doesn’t matter, but order matters.

ApprovedDate            ChangeDate                         changeByUser
NULL                    2019-12-09 06:40:15.437              vaisakh
NULL                    2019-12-09 06:42:31.563             vaisakh
NULL                    2019-12-09 06:42:33.140             vaisakh
NULL                    2019-12-09 07:03:54.660              vaisakh
2019-12-09 07:05:29.800 2019-12-09 07:05:29.817              vaisakh
2019-12-09 07:05:29.800 2019-12-09 07:05:38.707              vaisakh
NULL                    2019-12-09 07:09:33.160               vaisakh
NULL                    2019-12-09 07:09:42.440               vaisakh
NULL                    2019-12-09 09:38:19.757             vaisakh
2019-12-09 09:41:42.977 2019-12-09 09:41:43.243             Raveendran        

In this case I want first record and 5th record (Someone approved the data that’s why a value),then 7th record value is null someone rejected it.

I tried using recursive CTE it is working but for large records huge performance issue

DECLARE @today DATETIME = GETDATE();

with RESULT (CIPApprovedDate,ChangeDate,changeByUser,legalId,depth)AS(

  SELECT TOP 1 CIPApprovedDate, ChangeDate, changeByUser, legalId,1
  FROM LegalEntityExtensionLog
  WHERE legalId= 2688518
  ORDER BY ChangeDate ASC

  union ALL

  select 

  L.CIPApprovedDate, L.ChangeDate, L.changeByUser, L.legalId,ct.depth+1
  FROM LegalEntityExtensionLog L INNER JOIN Result CT
 on L.legalId=CT.legalId AND L.changeDate>CT.changeDate

 AND ISNULL(L.CIPApprovedDate,@today) <> ISNULL(CT.CIPApprovedDate,@today) 

)select * from Log  where  ChangeDate in(select MIN(ChangeDate) from Result group by depth)

Answer

You can pull out the transition records like so:

select ApprovedDate, ChangeDate, changeByUser
from (
    select 
        l.*,
        lag(ApprovedDate) ver(partition by LegalId order by ChangeDate) lagApprovedDate
    from LegalEntityExtensionLog l
) t
where 
    (lagApprovedDate is null and ApprovedDate is not null)
    or (lagApprovedDate is not null and ApprovedDate is null)

This will exhibit records where ApprovedDate transitioned from a null to a non null value (or the other way around).