On Mon, Mar 08, 2010 at 05:09:14PM +0100, Adrian von Bidder wrote: > Hi, > > The documentation about DROP COLUMN is a bit unclear: > > | The DROP COLUMN form does not physically remove the column, but simply > | makes it invisible to SQL operations. Subsequent insert and update > | operations in the table will store a null value for the column. Thus, > | dropping a column is quick but it will not immediately reduce the on-disk > | size of your table, as the space occupied by the dropped column is not > | reclaimed. The space will be reclaimed over time as existing rows are > | updated. > > "subsequent ... will store a null value" would imply that deleted columns > will still take some place, while "the space will be reclaimed ..." would > suggest that new rows (insert or updates in mvcc) don't have the deleted > column anymore - I'm not quite sure how to interpret this. What is pg > doing? What you're missing is that in postgres NULLs are stored as a bit in the header and there is no data. So in a sense NULLs take no space (well, one bit) which means both statements are true. Have a nice day, -- Martijn van Oosterhout <kleptog@xxxxxxxxx> http://svana.org/kleptog/ > Please line up in a tree and maintain the heap invariant while > boarding. Thank you for flying nlogn airlines.
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