Could someone help me understand the ambiguity here in Postgres? - sql

Question

So I've been trying out PG for a few days, specifically through NpgSQL in dotnet core, but I don't believe that is relevant to my question. I've been writing a couple of update functions. The first one was easy:
CREATE OR REPLACE FUNCTION "Api"."UpdateExpenseReceipt" ( "vReceiptID" UUID , "vTotal" DOUBLE PRECISION , "vTaxPercent" DOUBLE PRECISION , "vShippingCost" DOUBLE PRECISION , "vReceiptDate" TIMESTAMP , "vReference" VARCHAR , "vCurrentToken" UUID )
RETURNS TABLE ( ReceiptID UUID , Total DOUBLE PRECISION , TaxPercent DOUBLE PRECISION , ShippingCost DOUBLE PRECISION , Reference VARCHAR(96) , ReceiptDate TIMESTAMP )
LANGUAGE PLPGSQL
AS $$
DECLARE "iValidReceipt" INTEGER;
DECLARE "iValidUser" INTEGER;
BEGIN
"iValidReceipt" := ( SELECT COUNT("ReceiptID") FROM "Users"."ExpenseReceipt" WHERE "ReceiptID" = "vReceiptID" );
"iValidUser" := ( SELECT COUNT("AccountID") FROM "Users"."Account" WHERE "CurrentToken" = "vCurrentToken" LIMIT 1 );
IF "iValidUser" = 0 THEN
RAISE 'Error' USING ERRCODE = '10001';
END IF;
IF "iValidReceipt" > 0 THEN
UPDATE "Users"."ExpenseReceipt" SET
"Total" = COALESCE( "vTotal" , "Total" )
, "TaxPercent" = COALESCE( "vTaxPercent" , "TaxPercent" )
, "ShippingCost" = COALESCE( "vShippingCost" , "ShippingCost" )
, "Reference" = COALESCE( CAST( "vReference" AS VARCHAR ) , "Reference" )
, "ReceiptDate" = COALESCE( "vReceiptDate" , "ReceiptDate" )
, "EditDate" = current_timestamp at time zone 'utc'
WHERE "ReceiptID" = "vReceiptID";
RETURN QUERY
SELECT
"ReceiptID"
, "Total"
, "TaxPercent"
, "ShippingCost"
, "Reference"
, "ReceiptDate"
FROM "Users"."ExpenseReceipt"
WHERE "ReceiptID" = "vReceiptID";
ELSE
RAISE 'Error' USING ERRCODE = '10101';
END IF;
END; $$
--I'll include the table itself in case its relevant
CREATE TABLE "Users"."ExpenseReceipt"
(
"ReceiptID" UUID NOT NULL DEFAULT uuid_generate_v4(),
"AccountID" UUID NOT NULL ,
"Total" DOUBLE PRECISION NOT NULL ,
"TaxPercent" DOUBLE PRECISION DEFAULT 0.0 ,
"ShippingCost" DOUBLE PRECISION DEFAULT 0.0 ,
"Reference" VARCHAR(96) ,
"ReceiptDate" TIMESTAMP ,
"EditDate" TIMESTAMP DEFAULT ( current_timestamp at time zone 'utc' )
);
Easy update function, uses coalesce to not update the value if the API call doesn't set them. Everything works fine (after dealing through the numerous naming issues I've run into with postgres, and there I think NpgSQL is relevant). I know how to get it right. I made a second one, basically exactly the same:
CREATE OR REPLACE FUNCTION "Api"."UpdateSupplyItem" ( "vItemID" UUID , "vDescription" VARCHAR , "vSize" VARCHAR , "vNetCost" DOUBLE PRECISION , "vPackageQuantity" DOUBLE PRECISION , "vNetWeight" DOUBLE PRECISION , "vCurrentToken" UUID )
RETURNS TABLE ( "ItemID" UUID , "Description" VARCHAR , "Size" VARCHAR , "NetCost" DOUBLE PRECISION , "PackageQuantity" DOUBLE PRECISION , "NetWeight" DOUBLE PRECISION )
LANGUAGE PLPGSQL
AS $$
DECLARE "iValidItem" INTEGER;
DECLARE "iValidUser" INTEGER;
BEGIN
"iValidItem" := ( SELECT COUNT(usi."ItemID") FROM "Users"."SupplyItem" usi WHERE usi."ItemID" = "vItemID" );
"iValidUser" := ( SELECT COUNT("AccountID") FROM "Users"."Account" WHERE "CurrentToken" = "vCurrentToken" LIMIT 1 );
IF "iValidUser" = 0 THEN
RAISE 'Error' USING ERRCODE = '10001';
END IF;
IF "iValidItem" > 0 THEN
UPDATE "Users"."SupplyItem"
SET
"Description" = COALESCE( "vDescription" , "Users"."SupplyItem"."Description" )
, "Size" = COALESCE( "vSize" , "Users"."SupplyItem"."Size" )
, "NetCost" = COALESCE( "vNetCost" , "Users"."SupplyItem"."NetCost" )
, "PackageQuantity" = COALESCE( "vPackageQuantity" , "Users"."SupplyItem"."PackageQuantity")
, "NetWeight" = COALESCE( "vNetWeight" , "Users"."SupplyItem"."NetWeight" )
WHERE "Users"."SupplyItem"."ItemID" = "vItemID";
RETURN QUERY SELECT usi."ItemID" , usi."Description" , usi."Size" , usi."NetCost" , usi."PackageQuantity" , usi."NetWeight" FROM "Users"."SupplyItem" usi WHERE usi."ItemID" = "vItemID" LIMIT 1;
ELSE
RAISE 'Error' USING ERRCODE = '10401';
END IF;
END; $$
--Again, the table in case it helps
CREATE TABLE "Users"."SupplyItem"
(
"ItemID" UUID NOT NULL DEFAULT uuid_generate_v4() ,
"AccountID" UUID NOT NULL ,
"Description" VARCHAR ,
"Size" VARCHAR ,
"NetCost" DOUBLE PRECISION ,
"PackageQuantity" DOUBLE PRECISION ,
"NetWeight" DOUBLE PRECISION
);
but it's quite different. You can see clearly that I've had to fully qualify the right hand side of every equals (where the earlier function had no need). I get ambiguity all the way down the update statement. It starts at ItemID, then Description, then Size... every attribute I think. First thing I did was alias the table
UPDATE usi [...] FROM "Users"."SupplyItem" usi
but that failed because you can't do short aliasing in an UPDATE in PG (relation usi."[...]" does not exist") which actually kind of sucks. I only figured out that it needed to be fully qualified when someone asked a similar question and the answer was "It must be a quirk of RETURNS TABLE."
So why is my second update "a quirk" but my first update works perfectly? I've had a tough time with PG (and I'm not a slouch), but having two functions that seem identical having entirely different results (at runtime no less) makes me uncomfortable. I'm posting here because I know the two functions must be markedly different; 99.9% of the time, there is no such thing as a "quirk." There is something I need to understand to work around to avoid in the future. What is the "gotcha" that I've missed in the second UPDATE function?

Answer 1

The problem is that you have a function variable "Size" (an OUT parameter defined in the RETURNS TABLE clause) and a column "Size" in "Users"."SupplyItem". So you have to qualify the reference to indicate what you mean.
I recommend using an alias for simplicity:
UPDATE "Users"."SupplyItem" AS si
SET "Size" = COALESCE("vSize" , si."Size")
...
There is no such ambiguity in your first example, because you didn't double quote the parameter TaxPercent, so it gets case folded to taxpercent and is different from the column "TaxPercent".