Cache the columns that are found in the clickhouse table #228
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The singer sdk SQLConnector assumes that get_table_columns is a pretty quick operation because it loops over every column in the schema an calls it twice. Once for getting the type
_get_column_typeand once forcolumn_existsRoundtrip requests to clickhouse to get this information are not super fast, and tables with a large amount of columns can take a very long time to initialize.
This change caches the results of this column lookup so that we only have to inspect them once. It is possible that this operation is not safe if the sink is re-used because of meltano/sdk#2352 but given the simple use case that target clickhouse is using for this connector, this change is safe.