On 07/29/10 2:58 PM, Dino Vliet wrote:
Dear postgresql list,
I have some troubles generating data for a analysis task at hand.
I have a table (table A) containing 5 million records and 28 number of
attributes. This table is 461MB big if I copy it to a csv file.
I want to create another table (table B) based on the contents of
table A plus some 15 extra attributes (in pl/pgsql written functions
which produce those extra attributes)
So my statement looks like this:
create tableB as (
select some attributes, function1(A.attribute1)as attributeX+1,
function2(A.attribute1,A.Attribute2,A.attribute3,A.attribute4,A.attribute5)
as attribute X+2......function15(A.attribute1,A.attribute9) as
attributeX+15 from tableA as A)
This takes almost 60 hours to finish on my database server running
debian 5.0 with XFS as filesystem containing 4GB RAM. I'm using
postgresql server version 8.3 (but am seeing the same phenomena on my
FreeBSD 8.0 database server running postgresql 8.4 as well)
I arrived at 15 functions because I had 7 or 8 joins in the past and
saw that my disk was getting hid and I had heard someplace that RAM is
faster so I rewrote those 7 or 8 joins as functions in pl/pgsql. They
were just simple lookups, although some of the functions are looking
stuff up in tables containing 78000 records. However, I thought this
wouldn't be a problem because they are simple functions which look up
the value of one variable based on a parameter. 3 of the more special
functions are shown here:
...
1.
What can I do to let the creation of table B go faster?
2.
Do you think the use of indices (but where) would help me? I
didn't go that route because in fact I don't have a where clause
in the create table B statement. I could put indices on the
little tables I'm using in the functions.
3.
What about the functions? Should I code them differently?
4.
What about my server configuration. What could be done over there?
Thanks in advanced
certainly your lookup tables should have a index on the key you're using
to look up values. without said index, that 78000 row 'little' table
will have to be sequentially scanned for every one of your several
million rows.
with said indexes, you may find that just doing JOINs when you actually
use this data rather than creating a new table will work quite nicely.
you could use a VIEW to do the joins transparently on the fly.
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