Selecting multiple parameters from UDF or procedure

问题

I'm trying to compute multiple values and fetch them in a select clause. Whether its computed via UDF or procedure does not matter to me but I can't figure out how to do it in either way. I want to use it like this:

SELECT ID, BITMAP(ID) FROM X;

which then should return a table with columns ID, Bitset1, Bitset2. If possible it should be as performant as possible.

I have three versions currently (simplified, there's actual more computation):

Table UDF

CREATE FUNCTION TPCH.BITMAP(IN col BIGINT)
RETURNS table("BITSET1" bigint, "BITSET2" bigint)
AS BEGIN
    declare bitset1, bitset2 bigint;
    bitset1 = 1;
    bitset2 = 2;
    return select bitset1 as "BITSET1", bitset2 as "BITSET2" from sys.dummy;
END;

Scalar UDF

CREATE FUNCTION BITMAP(IN col BIGINT)
RETURNS bitset1 bigint, bitset2 bigint
AS BEGIN
    declare bitset1, bitset2 bigint;
    bitset1 = 1;
    bitset2 = 2;
END;

Procedure

create procedure BITMAP(in col BIGINT,
                        out bitsets table("BITSET1" bigint, "BITSET2" bigint))
as begin
    bitsets = select 1 as "BITSET1", 2 as "BITSET2" from sys.dummy;
end;

If I execute the select statement from above I get different exceptions:

For TUDF/Procedure: cannot use procedure or table function in select projection column or cannot use window function w/o OVER: P_BITMAP

For SUDF: This user defined function has multiple outputs, but this usage requires a single output. Specify a single output.

I figured out that for SUDF I can write it like this:

select ID, BITMAP(ID).bitset1, BITMAP(ID).bitset2 from X;

But this executes BITMAP(ID) twice which makes it slow. I want to fetch both values in one go.

Is this even possible to do in Hana or are there other options?

EDIT 1: for clarification the udfs and procedure usually depend on the input. I just return 1 and 2 in the example for simplicity.

回答1:

As discussed, in HANA 1 there exist the following limitation concerning user defined functions:

table functions don't accept values of joined tables as input parameters. I.e. they cannot be used to implement a "lateral join"
scalar functions will be called and executed for every instance occurring in the statement. Fetching multiple return parameters from equivalent calls will also call the function multiple times.
table functions and procedures cannot be used in the projection list of a SELECT statement (not HANA specific)

To apply a user defined function to many values in parallel, HANA 2 provides multiple new features, e.g.

MAP_MERGE function, to implement a map-reduce calling pattern
deterministic user-defined functions, that cache results, so that sub-sequent calls can use the precomputed result

add: a good example for the MAP_MERGE feature in HANA 2 can be found here: https://blogs.sap.com/2016/12/01/sap-hana-2.0-sps-0-new-developer-features-database-development/

来源：https://stackoverflow.com/questions/44314720/selecting-multiple-parameters-from-udf-or-procedure

标签

stored-procedures

hana

udf