[Bug]: ISNULL function returns no response in some scenarios

[Teletele-Lin]

What happened?

It looks like ISNULL is getting stuck when executing; it just hangs and returns nothing.

DECLARE @val VARCHAR(3) = NULL;
SELECT ISNULL(@val, 'Hello World');

Version

BABEL_5_X_DEV (Default)

Extension

babelfishpg_tsql (Default)

Which flavor of Linux are you using when you see the bug?

Ubuntu (Default)

Relevant log output

2026-02-02 12:20:46.832 CST [63829] ERROR:  Number of bytes for the field of varchar(n) exeeds max specified for the field.
2026-02-02 12:20:46.832 CST [63829] CONTEXT:  PL/tsql function inline_code_block line 2 at SQL statement
        TDS Protocol: Message Type: SQL BATCH, Phase: TDS_REQUEST_PHASE_PROCESS. Writing the Tds response to the socket
2026-02-02 12:20:46.832 CST [63829] LOG:  Unmapped error found. Code: 2600, Message: Number of bytes for the field of varchar(n) exeeds max specified for the field., File: tdstypeio.c, Line: 2766, Context: TDS
2026-02-02 12:20:46.832 CST [63829] CONTEXT:  TDS Protocol: Message Type: SQL BATCH, Phase: TDS_REQUEST_PHASE_PROCESS. Writing the Tds response to the socket
2026-02-02 12:21:21.105 CST [66613] LOG:  EOF on TDS socket

Code of Conduct

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[tanscorpio7]

I can reproduce this, looks like babelfish is not truncating the string properly before sending it to client.

-- Works
1> DECLARE @val VARCHAR(3) = NULL; SELECT ISNULL(@val, 'Hel');
2> GO
coalesce
--------
Hel     

(1 rows affected)

-- But with more characters it fails
1> DECLARE @val VARCHAR(3) = NULL; SELECT ISNULL(@val, 'Hell');
2> GO
coalesce
--------
Sqlcmd: Error: Internal error at ReadAndHandleColumnData (Reason: Error reading column data).
SqlState HY000, Protocol error in TDS stream

The leading arguments should be truncated to the first argument length.

[tanscorpio7]

Tracking this under BABEL-1001

[Teletele-Lin]

core cases：

DECLARE @c CHAR(2); SELECT ISNULL(@c, 'ABCD') AS [CHAR_Result]; --  'AB'， bbf hang

DECLARE @v VARCHAR(2); SELECT ISNULL(@v, 'ABCD') AS [VARCHAR_Result]; --  'AB' bbf hang

DECLARE @nc NCHAR(2); SELECT ISNULL(@nc, N'ABCD') AS [NCHAR_Result]; --  'AB' , bbf lost 

DECLARE @nv NVARCHAR(2); SELECT ISNULL(@nv, N'ABCD') AS [NVARCHAR_Result]; --  'AB', bbf lost

DECLARE @b BINARY(2); SELECT ISNULL(@b, 0x12345678) AS [BINARY_Result]; -- 0x1234

DECLARE @vb VARBINARY(2); SELECT ISNULL(@vb, 0x12345678) AS [VARBINARY_Result]; -- 0x1234, bbf no truncate

mroe types:

-- TIME type
DECLARE @time5_null TIME(5) = NULL;
DECLARE @time0_null TIME(0) = NULL;
DECLARE @time7_null TIME(7) = NULL;

SELECT 
    'TIME(7)' AS [type],
    ISNULL(@time7_null, '23:59:59.9999999') AS [result1], -- absolutely bug
    ISNULL(@time7_null, '00:00:00') AS [result2];


-- DATETIMEOFFSET type
DECLARE @dto_3_null DATETIMEOFFSET(3) = NULL;
DECLARE @dto_7_null DATETIMEOFFSET(7) = NULL;

SELECT 
    'DATETIMEOFFSET(7)' AS [type],
    ISNULL(@dto_7_null, SYSDATETIMEOFFSET()) AS [result1],
    ISNULL(@dto_7_null, '2023-12-31 23:59:59.9999999 +09:00') AS [result2]; -- bug of DATETIMEOFFSET type

-- DECIMAL/NUMERIC type
DECLARE @dec_10_2_null DECIMAL(10, 2) = NULL;
DECLARE @dec_5_3_null DECIMAL(5, 3) = NULL;
DECLARE @num_38_10_null NUMERIC(38, 10) = NULL;

SELECT 
    'NUMERIC(38,10)' AS [type],
    ISNULL(@num_38_10_null, 123456789012345678.1234567890) AS [result1],
    ISNULL(@num_38_10_null, 1.234567890123456789) AS [result2] -- bbf error, mssql ok

SELECT 
    'DECIMAL(5,3)' AS [type],
    ISNULL(@dec_5_3_null, 12.345) AS [result1],
    ISNULL(@dec_5_3_null, 12.345678) AS [result2],
    ISNULL(@dec_5_3_null, 123.456) AS [result3]; --  msqql error, bbf ok