register_vector() Fails When Only 'vector' Extension is Installed, Missing 'halfvec' and 'sparsevec #79

Description:

After the 0.3.0 release, we have encountered an issue where the register_vector function throws an error if our PostgreSQL database only has the vector extension installed, and not halfvec or sparsevec.

Steps to Reproduce:

Install the pgvector extension in PostgreSQL without halfvec or sparsevec.
Call the register_vector function.
Error:

  File "/usr/local/lib/python3.9/site-packages/pgvector/psycopg2/register.py", line 23, in register_vector
    cur.execute('SELECT NULL::sparsevec')
psycopg2.errors.InFailedSqlTransaction: current transaction is aborted, commands ignored until end of transaction bloc

Expected Behavior:

The function should not break if halfvec or sparsevec are not present. It should work seamlessly with only the vector extension installed.

Link to Relevant Code:

register_vector