BUG #19033: Inconsistency between prepared statement and normal statement when cast bit to integer
The following bug has been logged on the website:
Bug reference: 19033
Logged by: Chi Zhang
Email address: 798604270@qq.com
PostgreSQL version: 18beta3
Operating system: ubuntu 24.04 with docker
Description:
Hi,
In the normal statement, casting a bit value to INT is allowed, but in a
prepared statement, it will trigger an error.
In the following test case, the first query will return 15, but the EXECUTE
statement will trigger an error: parameter $1 of type bit cannot be coerced
to the expected type integer
```
SELECT (B'1111')::INT;
int4
------
15
(1 row)
PREPARE prepare_query (unknown) AS SELECT $1::INT;
EXECUTE prepare_query(B'1111');
ERROR: parameter $1 of type bit cannot be coerced to the expected type
integer
```
If I modify the unknown type to bit, then the query can execute successfully
and return 15.
```
PREPARE prepare_query (bit) AS SELECT $1::INT;
EXECUTE prepare_query(B'1111');
int4
------
15
(1 row)
```
I also find if I first cast B'1111' to bit and then cast it to integer, it
will become 1. But the correct result should be 15.
```
SELECT ((B'1111')::bit)::INT;
int4
------
1
(1 row)
```
On Wednesday, August 27, 2025, PG Bug reporting form <noreply@postgresql.org>
wrote:
The following bug has been logged on the website:
Bug reference: 19033
Logged by: Chi Zhang
Email address: 798604270@qq.com
PostgreSQL version: 18beta3
Operating system: ubuntu 24.04 with docker
Description:Hi,
In the normal statement, casting a bit value to INT is allowed, but in a
prepared statement, it will trigger an error.In the following test case, the first query will return 15, but the EXECUTE
statement will trigger an error: parameter $1 of type bit cannot be coerced
to the expected type integer```
SELECT (B'1111')::INT;
int4
------
15
(1 row)PREPARE prepare_query (unknown) AS SELECT $1::INT;
EXECUTE prepare_query(B'1111');
ERROR: parameter $1 of type bit cannot be coerced to the expected type
integer
```If I modify the unknown type to bit, then the query can execute
successfully
and return 15.
```
PREPARE prepare_query (bit) AS SELECT $1::INT;
EXECUTE prepare_query(B'1111');
int4
------
15
(1 row)
```
This doesn’t feel like a bug, using “unknown” is likely to cause typing
issue, but I’m not sure I can explains exactly why.
I also find if I first cast B'1111' to bit and then cast it to integer, it
will become 1. But the correct result should be 15.```
SELECT ((B'1111')::bit)::INT;
int4
------
1
(1 row)
```
The documentation explicitly contradicts your claim here and explain why
this is correct.
David J.
PG Bug reporting form <noreply@postgresql.org> writes:
PREPARE prepare_query (unknown) AS SELECT $1::INT;
EXECUTE prepare_query(B'1111');
ERROR: parameter $1 of type bit cannot be coerced to the expected type
integer
The reason that fails is that EvaluateParams uses assignment-coercion
semantics, which is more limited in what it will allow than an
explicit coercion. I think this is correct behavior. The system
should not silently perform cross-type-category casts --- too much
risk of unintended results.
regards, tom lane