Hi,
I'm using Postgres 7.4.5. Tables are analyzed & vacuumed.
I am wondering why postgresql never uses an index on queries of the type
'select distinct ...' while e.g. mysql uses the index on the same query.
See the following explains:
postgresql:
explain analyze select distinct "land" from "customer_dim";
---------------------------------------------------------------------------------------------------------------------------------------+
QUERY PLAN
|
---------------------------------------------------------------------------------------------------------------------------------------+
Unique (cost=417261.85..430263.66 rows=18 width=15) (actual time=45875.235..67204.694 rows=103 loops=1)
|
-> Sort (cost=417261.85..423762.75 rows=2600362 width=15) (actual time=45875.226..54114.473 rows=2600362 loops=1)
|
Sort Key: land
|
-> Seq Scan on customer_dim (cost=0.00..84699.62 rows=2600362 width=15) (actual time=0.048..10733.227
rows=2600362loops=1) |
Total runtime: 67246.465 ms
|
---------------------------------------------------------------------------------------------------------------------------------------+
mysql:
explain select DISTINCT `customer_dim`.`land` from `customer_dim`;
--------------+-------+---------------+---------------+---------+--------+---------+-------------+
table | type | possible_keys | key | key_len | ref | rows | Extra |
--------------+-------+---------------+---------------+---------+--------+---------+-------------+
customer_dim | index | [NULL] | IDX_cstd_land | 81 | [NULL] | 2600362 | Using index |
--------------+-------+---------------+---------------+---------+--------+---------+-------------+
1 row in result (first row: 8 msec; total: 9 msec)
The result set contains 103 rows (but i get this behavior with every query of
this kind). My tables consist of at least a million rows.
The indexes on the column 'land' are standard indexes, so in case of
postgresql, it's a btree-index. I've tried to change the index type, but to no
avail.
So, why doesn't postgresql use the index, and (how) could i persuade postgresql
to use an index for this type of query?
TiA
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Ole Langbehn
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