Обсуждение: index-only-scan when there is an index on all columns

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index-only-scan when there is an index on all columns

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Hadi Moshayedi

Дата:

27 февраля 2018 г., 23:37:28

With the following definition:

create table t1 (a int, b int, c int);
create index on t1 (a,b,c);

I get the following plan for the following query:

postgres=# explain select sum(c) from t1 where a > 0;
                                    QUERY PLAN
-----------------------------------------------------------------------------------
Aggregate (cost=29.62..29.63 rows=1 width=8)
   -> Bitmap Heap Scan on t1 (cost=9.42..27.92 rows=680 width=4)
         Recheck Cond: (a > 0)
         -> Bitmap Index Scan on t1_a_b_c_idx (cost=0.00..9.25 rows=680 width=0)
               Index Cond: (a > 0)
(5 rows)

I am wondering why is it not using index-only-scan (which would use the cache better) and instead it does a bitmap scan?

Thanks,

Hadi

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Обсуждение: index-only-scan when there is an index on all columns

index-only-scan when there is an index on all columns