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PostgreSQL, UPDATE CASE statement uses huge amount of space

I am running a CASE statement on one of my tables and it consumes a huge amount of hard drive space that is never freed up even when i run VACUUM ANALYZE. Autovacuum is ON as well.

I am using PostgreSQL 9.3 64Bit on a windows 2012 server. I have tweaked the postgresql.conf using advice on the wiki. So it is as optimised as I can make it.

The table is large (>150million rows) which I need to add an extra column and populate it based on the contents of 3 other columns. The size of this table is 53Gb without indexes.

Having tested lots of approaches I am using a CASE statement. Two of the columns are arrays so I have used GIN indexes and a standard index.

A sample of my CASE statement looks like this, the rules go up to 38

UPDATE osmm.topographicarea
    SET fcode = (CASE
        --building CASE statements
        WHEN (descriptivegroup @> '{Building}' and descriptiveterm @> '{Archway}') then 1
        WHEN (descriptivegroup @> '{Building}') then 2

        WHEN (descriptiveterm @> '{Step}') then 3
        WHEN (descriptivegroup @> '{Glasshouse}') then 4
        WHEN (descriptivegroup @> '{Historic Interest}') then 5
        WHEN (descriptivegroup @> '{Inland Water}') then 6
      ELSE 99             
   END);
COMMIT;

The process takes over 5hrs but adds a huge 180Gb to the table!!

Adding an integer column to this table surely should not do this?

How can I fix this please?