Best way to compress xml text column using SQL? - sql

Question

Using Microsoft SQL Server 2019.
I have two columns, one text representing some xml, another varbinary(max) representing already compressed xml, that I need to compress.
Please assume I cannot change the source data, but conversions can be made as necessary in the code.
I'd like to compress the text column, and initially it works fine, but if I try to save it into a temp table to be used further along in the process I get weird characters like ‹ or tŠÌK'À3û€Í‚;jw. Again, the first temp table I make stores it just fine, I can select the initial table and it displays compressed correctly. But if I need to pull it into a secondary temp table or variable from there it turns into a mess.
I've tried converting into several different formats, converting later in the process, and bringing in the source data for the column at the very last stage, but my end goal is to populate a variable that will be converted into JSON, and it always ends up weird there as well. i just need the compressed version of the columns do display properly when viewing the json variable I've made.
Any suggestions on how to tackle this?

Answer 1

Collation issue?
This smells of collation issue. tempdb is actually its own database with its own default collation and other settings.
In one database with default CollationA you call COMPRESS(NvarcharData) and that produces some VARBINARY.
In other database (tempdb) with default CollationB you call CONVERT(NVARCHAR(MAX), DECOMPRESS(CompressedData)). Now, what happens under the hood is:
CompressedData gets decompressed into VARBINARY representing NvarcharData in CollationA
that VARBINARY is converted to NVARCHAR assuming the binary data represents NVARCHAR data in CollationB, which is not true!
Try to be more explicit (collation, data type) with conversions between XML, VARBINARY and (N)VARCHAR.
Double compression?
I have also noticed "representing already compressed xml, that I need to compress". If you are doublecompressing, maybe you forgot to doubledecompress?
Example?
You are sadly missing an example, but I have produced minimal example of converting between XML and compressed data that works for me.
BEGIN TRANSACTION
GO
CREATE TABLE dbo.XmlData_Base (
PrimaryKey INTEGER NOT NULL IDENTITY(1, 1),
XmlCompressed VARBINARY(MAX) NULL
);
GO
CREATE OR ALTER VIEW dbo.XmlData
WITH SCHEMABINDING
AS
SELECT
BASE.PrimaryKey,
CONVERT(XML, DECOMPRESS(BASE.XmlCompressed)) AS XmlData
FROM
dbo.XmlData_Base AS BASE;
GO
CREATE OR ALTER TRIGGER dbo.TR_XmlData_instead_I
ON dbo.XmlData
INSTEAD OF INSERT
AS
BEGIN
INSERT INTO dbo.XmlData_Base
(XmlCompressed)
SELECT
COMPRESS(CONVERT(VARBINARY(MAX), I.XmlData))
FROM
Inserted AS I;
END;
GO
CREATE OR ALTER TRIGGER dbo.TR_XmlData_instead_U
ON dbo.XmlData
INSTEAD OF UPDATE
AS
BEGIN
UPDATE BASE
SET
BASE.XmlCompressed = COMPRESS(CONVERT(VARBINARY(MAX), I.XmlData))
FROM
dbo.XmlData_Base AS BASE
JOIN Inserted AS I ON I.PrimaryKey = BASE.PrimaryKey;
END;
GO
INSERT INTO dbo.XmlData
(XmlData)
VALUES
(CONVERT(XML, N'<this><I>I call upon thee!</I></this>'));
SELECT
*
FROM
dbo.XmlData;
SELECT
PrimaryKey,
XmlCompressed,
CONVERT(XML, DECOMPRESS(XmlCompressed))
FROM
dbo.XmlData_Base;
UPDATE dbo.XmlData
SET
XmlData = CONVERT(XML, N'<that><I>I call upon thee!</I></that>');
SELECT
*
FROM
dbo.XmlData;
SELECT
PrimaryKey,
XmlCompressed,
CONVERT(XML, DECOMPRESS(XmlCompressed))
FROM
dbo.XmlData_Base;
GO
ROLLBACK TRANSACTION;