Re: What does Rows Counted mean - Mailing list pgsql-general

This is on the object browser. When you navigate to the table and
click on the table name, you will see the output on the right side and
check the Properties tab.

On Tue, Apr 5, 2016 at 10:37 AM, Adrian Klaver
<adrian.klaver@aklaver.com> wrote:
> On 04/05/2016 07:11 AM, Rakesh Kumar wrote:
>>
>> pgAdmin shows this:
>>
>> Name:   tableA
>> OID
>> Owner
>> Tablespace
>> Rows (estimated) : 100000
>> Rows (Counted)   : not counted
>>
>> What is Rows (Counted) and why it is showing not counted even though
>> the table has been analyzed.
>
>
> Where is the above coming from in pgAdmin?
>
> http://www.postgresql.org/docs/9.5/interactive/sql-analyze.html
>
> "ANALYZE collects statistics about the contents of tables in the database,
> and stores the results in the pg_statistic system catalog. "
>
> so:
>
> production=# analyze verbose projection;
> INFO:  analyzing "public.projection"
> INFO:  "projection": scanned 403 of 403 pages, containing 25309 live rows
> and 0 dead rows; 25309 rows in sample, 25309 estimated total rows
> ANALYZE
>
>
> Therefore the results of ANALYZE are snapshots in time and are considered to
> be estimates.
>
> EXPLAIN ANALYZE gives you an actual count:
>
> http://www.postgresql.org/docs/9.5/interactive/sql-explain.html
>
> production=# explain analyze select count(*) from projection where p_item_no
> < 100;
>                                                           QUERY PLAN
> ------------------------------------------------------------------------------------------------------------------------------
>  Aggregate  (cost=438.72..438.73 rows=1 width=0) (actual time=1.655..1.655
> rows=1 loops=1)
>    ->  Bitmap Heap Scan on projection  (cost=13.74..436.96 rows=703 width=0)
> (actual time=0.328..1.226 rows=679 loops=1)
>          Recheck Cond: (p_item_no < 100)
>          Heap Blocks: exact=120
>
>          ->  Bitmap Index Scan on pj_pno_idx  (cost=0.00..13.56 rows=703
> width=0) (actual time=0.271..0.271 rows=679 loops=1)
>                Index Cond: (p_item_no < 100)
>
>  Planning time: 0.181 ms
>
>  Execution time: 1.749 ms
>
> (8 rows)
>
>
>
>
> production=# select count(*) from projection where p_item_no < 100;
>  count
>
> -------
>
>    679
>
> (1 row)
>
>
> But, again that is a snapshot of a point in time:
>
>
> production=# begin;
> BEGIN
> production=# delete from projection where p_item_no < 25;
> DELETE 117
> production=# explain analyze select count(*) from projection where p_item_no
> < 100;
>                                                           QUERY PLAN
> ------------------------------------------------------------------------------------------------------------------------------
>  Aggregate  (cost=438.72..438.73 rows=1 width=0) (actual time=1.517..1.518
> rows=1 loops=1)
>    ->  Bitmap Heap Scan on projection  (cost=13.74..436.96 rows=703 width=0)
> (actual time=0.336..1.159 rows=562 loops=1)
>          Recheck Cond: (p_item_no < 100)
>          Heap Blocks: exact=120
>          ->  Bitmap Index Scan on pj_pno_idx  (cost=0.00..13.56 rows=703
> width=0) (actual time=0.271..0.271 rows=679 loops=1)
>                Index Cond: (p_item_no < 100)
>  Planning time: 0.214 ms
>  Execution time: 1.610 ms
> (8 rows)
>
> production=# select count(*) from projection where p_item_no < 100;
>  count
> -------
>    562
> (1 row)
>
> Note the difference in actual row count between the Bitmap Index Scan and
> the Bitmap Heap Scan, due to the above taking place in an open transaction
> where the 117 'deleted' rows are still in play until I either commit or
> rollback.
>
>
> Unless the table is getting absolutely no activity a row count is going to
> be tied to a point in time.
>
>>
>> thanks
>>
>>
>
>
> --
> Adrian Klaver
> adrian.klaver@aklaver.com


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