Why does Postgresql use a Seq Scan on the first column of the PK?

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postgresql query-performance index-tuning primary-key seqscan

                          This is my table:

    CREATE TABLE IF NOT EXISTS public.ob_samples_sc5555 (
        stamp timestamp(6) with time zone NOT NULL,
        oblvl smallint NOT NULL,
        olots integer NOT NULL,
        CONSTRAINT prk_ob_samples_sc5555 PRIMARY KEY (stamp, oblvl)
    );

This is my query:

    delete from ob_samples_sc5555
    where stamp    Finalize Aggregate  (cost=10106164.53..10106164.54 rows=1 width=8)
>   
>     ->  Gather  (cost=10106163.59..10106164.50 rows=9 width=8)
>           Workers Planned: 9
>           ->  Partial Aggregate  (cost=10105163.59..10105163.60 rows=1 width=8)
>                 ->  Parallel Seq Scan on ob_samples_sc5555  (cost=0.00..9917330.49 rows=75133240 width=0)
>                       Filter: (stamp    JIT:
>     Functions: 6
>     Options: Inlining true, Optimization true, Expressions true, Deforming true

Why does PG13 use a seq scan instead of primary key, while the column stamp is the first column of the PK?



                        

Asked by Leon (411 rep)

Feb 20, 2025, 01:20 AM

Why does Postgresql use a Seq Scan on the first column of the PK?

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