Question regarding the output of postgresql *explain* command

[Arup Rakshit <aruprakshit1987@xxxxxxxxxxx>]

Hi,

I was reading to day how indexing works. And I was trying some query, for example below one. What the range basically means (cost=0.28..8.30 ? I don’t understand this.

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arup@ror ~/part-time-projects/entrylvl (add_index_to_job_sources)$ rails db
psql (9.5.0)
Type "help" for help.

entrylvl_dev=# \d job_sources;
                                       Table "public.job_sources"
    Column     |            Type             |                        Modifiers
---------------+-----------------------------+----------------------------------------------------------
 id            | bigint                      | not null default nextval('job_sources_id_seq'::regclass)
 job_id        | bigint                      |
 source        | character varying           |
 job_source_id | character varying           |
 created_at    | timestamp without time zone | not null
 updated_at    | timestamp without time zone | not null
Indexes:
    "job_sources_pkey" PRIMARY KEY, btree (id)
    "index_job_sources_on_source_and_job_source_id" UNIQUE, btree (source, job_source_id)
    "index_job_sources_on_job_id" btree (job_id)
Foreign-key constraints:
    "fk_rails_f45da00eca" FOREIGN KEY (job_id) REFERENCES jobs(id)

entrylvl_dev=# explain select * from job_sources where job_source_id = 'p_1208b146c9eb8905' AND source = 'indeed';
                                                    QUERY PLAN
------------------------------------------------------------------------------------------------------------------
 Index Scan using index_job_sources_on_source_and_job_source_id on job_sources  (cost=0.28..8.30 rows=1 width=58)
   Index Cond: (((source)::text = 'indeed'::text) AND ((job_source_id)::text = 'p_1208b146c9eb8905'::text))
(2 rows)

entrylvl_dev=#

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Thanks,

Arup