OdbcConnection.GetSchema("Columns") always returns NULL in COLUMN_DEF for Jet ODBC driver

[lauxjpn]

An OdbcConnection.GetSchema("Columns") call using the Jet ODBC driver always returns NULL for COLUMN_DEF, even if the column has a default value defined in the Jet database.

The same call on an OLE DB connection works as expected and correctly returns the default value in its COLUMN_DEFAULT column.

This might be a bug in the underlying Jet ODBC driver implementation for SQLColumns() (I have not checked this).

/cc @saurabh500

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[lauxjpn]

BTW, I came across more issues in DAO, ADO and also in OLE DB while I was implementing the schema reader that EntityFrameworkCore.Jet now uses for EF Core 3.1 to scaffold databases from.

Some are bugs, some are Jet features implemented in one technology but missing support in others and some are just common features missing in a technology.

We worked around all of them by switching between data access technologies (or using a combination of both, DAO and ADO) depending on what part of the schema we need to scaffold at the moment, but at some point, assuming Access stays supported by Microsoft, those issues should be resolved.

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