how to use all the options of EXPLAIN?

On 15/09/11 15:43, AI Rumman wrote:

Could anyone please tell me how to use all the options of explain in
Postgresql 9.
http://www.postgresql.org/docs/9.0/static/sql-explain.html

I can use EXPLAIN ANALYZE.
explain (format yaml) select * from tab1;

But

explain (format yaml) analyze select * from tab1;
ERROR: syntax error at or near "analyze"
LINE 1: explain (format yaml) analyze select * from tab1

BUFFERS:

explain (buffers true) select * from tab1;
ERROR: EXPLAIN option BUFFERS requires ANALYZE

But

explain (buffers true) analyze select * from tab1;
ERROR: syntax error at or near "analyze"
LINE 1: explain (buffers true) analyze select * from tab1...

Same for COSTS.

Does any of our experts use these options? If yes, please tell me how.

Try putting the ANALYZE inside the parentheses..
eg
EXPLAIN (buffers true, analyze) select * from tab1;

When I am using the following query, I got shared hit=8857.

EXPLAIN (buffers true, analyze) select * from act;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------
Seq Scan on act (cost=0.00..6308.82 rows=542312 width=735) (actual
time=0.007..516.199 rows=542312 loops=1)
Buffers: shared hit=8857
Total runtime: 1004.067 ms

Wherears, when I am using the following query, I got Buffers: shared
hit=416 read=46350

EXPLAIN (buffers true, analyze) select * from crm;

QUERY
PLAN
-------------------------------------------------------------------------------------------------------------------------------
Seq Scan on crm (cost=0.00..21558.94 rows=1688234 width=333) (actual
time=0.040..1738.903 rows=1688291 loops=1)
Buffers: shared hit=416 read=46350
Total runtime: 3257.872 ms

Could you please tell me why the different that is why I get read for one
query and not for others?

Thanks.

On Thu, Sep 15, 2011 at 11:59 AM, Toby Corkindale <
toby.corkindale@strategicdata.com.au> wrote:

On Thu, 2011-09-15 at 12:17 +0600, AI Rumman wrote:

When I am using the following query, I got shared hit=8857.

EXPLAIN (buffers true, analyze) select * from act;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------
Seq Scan on act (cost=0.00..6308.82 rows=542312 width=735) (actual
time=0.007..516.199 rows=542312 loops=1)
Buffers: shared hit=8857
Total runtime: 1004.067 ms

Wherears, when I am using the following query, I got Buffers: shared
hit=416 read=46350

EXPLAIN (buffers true, analyze) select * from crm;

QUERY
PLAN
-------------------------------------------------------------------------------------------------------------------------------
Seq Scan on crm (cost=0.00..21558.94 rows=1688234 width=333) (actual
time=0.040..1738.903 rows=1688291 loops=1)
Buffers: shared hit=416 read=46350
Total runtime: 3257.872 ms

Could you please tell me why the different that is why I get read for one
query and not for others?

The act table was in PostgreSQL cache memory, so it reads it from there.
A small part of the crm table was in PostgreSQL cache memory, it read
this part from there, but had to ask the OS to get the rest of the table
from the OS memory or from disk.

--
Guillaume
http://blog.guillaume.lelarge.info
http://www.dalibo.com