Dimension columns

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Dimension columns identify a given dimension.

Dimension column	Type	Description	Applicable dimensions
`date`	`Date`	A date-time, indicating the start of the reporting period for a given row.	`by_item_by_smallest_time_bucket`
`item_id`	`Number`	The numeric Item ID.	`by_item` `by_item_by_smallest_time_bucket`
`item_type`	`String`	Possible values: `"ITEM"` (a regular item) `"RSS"` (see: RSS Items)	`by_item` `by_item_by_smallest_time_bucket`
`description`	`String`	The description for the item (ad).	`by_item` `by_item_by_smallest_time_bucket`
`thumbnail_url`	String	The URL for the item (ad) thumbnail.	`by_item` `by_item_by_smallest_time_bucket`
`url`	String	The target URL for the item (ad).	`by_item` `by_item_by_smallest_time_bucket`

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The 2 dimensions have nearly identical dimension columns.