Varbinary(max) parameters in Nested Stored Procedures by value or reference _Expand / Collapse

Question

Consider a situation where a stored procedure taking a varbinary(max) (BLOB) input parameter
then calls a nested stored procedure and passes along that varbinary(max) as an input parameter to the nested stored procedure.
Is a copy of the BLOB provided to the nested stored procedure (passed by value) OR is the BLOB passed by reference.
My interest is in understanding the potential memory hit when handling large BLOBs in this environment.
For example, if the BLOB is 200MB, will SQL server need to allocate memory for a new copy each time it's passed to another stored procedure at the next nestlevel?
Looks like table type parameters are passed by reference, but I haven't been able to find any info on BLOBS in this context.

Answer 1

The semantics for parameters in SQL Server is copy-in/copy-out. However, this does not mean that there cannot be optimizations. That is, there is no need to copy the BLOB, as long as the procedure does not change it. Whether they actually have such an optimization,
I don't know.
I composed the repro below, and it is sort of interesting. If I restart my instance (SQL 2014), the memory usage grows with about 40 M for each nesting call, which certainly indicates that the BLOB is copied over and over again. But if I then run it again,
the growth is not the same. This may be because, it uses buffers already available. (A BLOB has to be a handle like a table when it is over some size.) Then again, it means that it is squeezing something else out of the cache.
CREATE PROCEDURE K  @h varchar(MAX) AS
   SELECT SUM(pages_kb)*8192 AS K, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   SELECT @h = ''
go
CREATE PROCEDURE L  @h varchar(MAX) AS
   SELECT SUM(pages_kb)*8192 AS L1, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   EXEC K @h
   SELECT SUM(pages_kb)*8192 AS L2, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   --SELECT @h = ''
go
CREATE PROCEDURE M  @h varchar(MAX) AS
   SELECT SUM(pages_kb)*8192 AS M1, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   EXEC L @h
   SELECT SUM(pages_kb)*8192 AS M2, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   --SELECT @h = ''
go
CREATE PROCEDURE N  @h varchar(MAX) AS
   SELECT SUM(pages_kb)*8192 AS N1, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   EXEC M @h
   SELECT SUM(pages_kb)*8192 AS N2, datalength(@h) as Dlen FROM sys.dm_os_memory_clerks
   --SELECT @h = ''
go
DECLARE @h varchar(MAX) = replicate(cast('ABCD' AS varchar(MAX)), 1E7)
EXEC N @h
go
DROP PROCEDURE K, L, M, N
Erland Sommarskog, SQL Server MVP, [email protected]