Fixed bulkcopy metadata query to make use of cached data if available

[tkyc]

Prior to this PR, the column metadata query call was made every single time despite data being cached. We now only make the call if no data is cached.

Fixes #2224.

[mmimica]

When is metadata actually being reused?

From what I see, SQLServerBulkCopy#getDestinationMetadata is only ever called once from SQLServerPreparedStatement#execute[Large]Batch, where a new SQLServerBulkCopy object is created for each call.

I think the point of SQLServerBulkCopy#destColumnMetadata is not re-use, but merely having the state around to avoid passing it around as a method argument.

[tkyc]

When is metadata actually being reused?

The wording on the issue can probably be corrected/improved on my end. Originally, the problem scenario occurs when a user repeatedly calls SQLServerBulkCopy#writeToServer(ISQLServerBulkData sourceData) (for the same SQLServerBulkCopy object). Doing so, the metadata query call would be repeated as well and the destColumnMetadata would be repopulated each time which impacted performance. After this PR fix, the subsequent bulk inserts wouldn't make the metadata query call and would "reuse" the existing destColumnMetadata that was set from a prior writeToServer call. Feel free to Let me know of better phrasing or wording to convey the problem/fix.

From what I see, SQLServerBulkCopy#getDestinationMetadata is only ever called once from SQLServerPreparedStatement#execute[Large]Batch, where a new SQLServerBulkCopy object is created for each call.

Is this on the main branch of the driver project? On my end I see, SQLServerBulkCopy#getDestinationMetadata being called from SQLServerBulkCopy#writeToServer(), which is then called by SQLServerBulkCopy#writeToServer(ISQLServerBulkData sourceData). Let me know if I'm incorrect or missing any details here. Thanks.

[mmimica]

Are you saying users use SQLServerBulkCopy class directly? In that case it makes sense, yes.

[tkyc]

Are you saying users use SQLServerBulkCopy class directly?

That's right.