Dear All,
I have several tables containing data sorted by 2 keys (neither are keys in db
terms (not unique), however). I would like to retrieve all rows from all tables
sorted by the same keys, essentially merging the contents of the tables
together. While I am completely aware of sort order not being a (fundamental)
property of an RDBMS table, I am also aware of indices and clustering (in fact,
data is inserted into the tables into the correct order, and not consequently
modified in any way). I have a union query like this one:
select a,b,c,d,e from table1 union all
select a,b,c,d,e from table2 union all
etc...
select a,b,c,d,e from tablen order by a,b;
Is there a way to prevent PostgreSQL from doing a full sort on the result set
after the unions have been completed? Even if I write
(select a,b,c,d,e from table1 order by a,b) union all
(select a,b,c,d,e from table2 order by a,b) union all
etc...
(select a,b,c,d,e from tablen order by a,b) order by a,b;
PostgreSQL does not seem to realise (maybe it should not be able to do this
trick anyway) that the last "order by" clause is merely a final merge step on
the ordered data sets.
Is there a workaround for this within PostgreSQL (another type of query,
parameter tuning, stored procedure, anything) or should I use my back-up plan
of making separate queries and merging the results in the target language?
Thanks a lot,
Ambrus
--
Wagner, Ambrus (IJ/ETH/GBD)
Tool Designer
GSDC Hungary
Location: Science Park, A2 40 008
Phone: +36 1 439 5282
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