Re: [PERFORM] limit order by performance issue
Title: Documento sin título For this query: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; This is what EXPLAIN returns (can't make EXPLAIN ANALYZE because it "never" ends): "Limit (cost=0.00..258.58 rows=1 width=16)" " - Index Scan Backward using ism_floatvalues_index_time_stamp on ism_floatvalues (cost=0.00..8912076.82 rows=34466 width=16)" " Filter: (id_signal = 29660)" This is EXPLAIN ANALYZE without "limit 1": "Sort (cost=93683.39..93769.56 rows=34466 width=16) (actual time=188.643..188.650 rows=1 loops=1)" " Sort Key: time_stamp" " Sort Method: quicksort Memory: 17kB" " - Index Scan using ism_floatvalues_index on ism_floatvalues (cost=0.00..90494.38 rows=34466 width=16) (actual time=188.019..188.030 rows=1 loops=1)" " Index Cond: (id_signal = 29660)" "Total runtime: 189.033 ms" Note that I have created two indexes, the first on id_signal and the second on time_stamp. Regards. El 16/10/2012 21:23, Pavel Stehule escribió: 2012/10/15 Pedro Jiménez p.jime...@ismsolar.com: Hello, I'm trying to do a simple SQL query over Postgresl 9.0 running on Ubuntu. I have a large table (over 100 million records) with three fields, id_signal (bigint), time_stamp (timestamp) and var_value (float). My query looks like this: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; So I want to select the last value from a determinated ID (is_signal). This query runs FOREVER, while if I delete "limit 1" it runs instantly did you ANALYZE your tables? Can you send EXPLAIN ANALYZE result of both queries? Regards Pavel Stehule Any help? Regards. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance -- Pedro Jiménez Pérez p.jime...@ismsolar.com Innovación en Sistemas de Monitorización, S.L. Edificio Hevimar C/ Iván Pavlov 2 y 4 - Parcela 4 2ª Planta Local 9 Parque Tecnológico de Andalucía 29590 Campanillas (Málaga) Tlfno. 952 02 07 13 conta...@ismsolar.com Antes de imprimir, piensa en tu responsabilidad y compromiso con el MEDIO AMBIENTE! Before printing, think about your responsibility and commitment with the ENVIRONMENT! CLÁUSULA DE CONFIDENCIALIDAD.- Este mensaje, y en su caso, cualquier fichero anexo al mismo, puede contener información confidencial o legalmente protegida (LOPD 15/1999 de 13 de Diciembre), siendo para uso exclusivo del destinatario. No hay renuncia a la confidencialidad o secreto profesional por cualquier transmisión defectuosa o errónea, y queda expresamente prohibida su divulgación, copia o distribución a terceros sin la autorización expresa del remitente. Si ha recibido este mensaje por error, se ruega lo notifique al remitente enviando un mensaje al correo electrónico conta...@ismsolar.com y proceda inmediatamente al borrado del mensaje original y de todas sus copias. Gracias por su colaboración.
Re: [PERFORM] limit order by performance issue
On Wed, Oct 17, 2012 at 6:14 AM, Pedro Jiménez Pérez p.jime...@ismsolar.com wrote: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; This is what EXPLAIN returns (can't make EXPLAIN ANALYZE because it never ends): Limit (cost=0.00..258.58 rows=1 width=16) - Index Scan Backward using ism_floatvalues_index_time_stamp on ism_floatvalues (cost=0.00..8912076.82 rows=34466 width=16) Filter: (id_signal = 29660) This is EXPLAIN ANALYZE without limit 1: Add (or modify the existing) an index on id_signal, time_stamp desc, and you're done. It must be a case of descending time stamps not hitting the filter condition (id_signal) soon enough.
[PERFORM] limit order by performance issue
Hello, I'm trying to do a simple SQL query over Postgresl 9.0 running on Ubuntu. I have a large table (over 100 million records) with three fields, id_signal (bigint), time_stamp (timestamp) and var_value (float). My query looks like this: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; So I want to select the last value from a determinated ID (is_signal). This query runs FOREVER, while if I delete limit 1 it runs instantly Any help? Regards. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] limit order by performance issue
2012/10/15 Pedro Jiménez p.jime...@ismsolar.com: Hello, I'm trying to do a simple SQL query over Postgresl 9.0 running on Ubuntu. I have a large table (over 100 million records) with three fields, id_signal (bigint), time_stamp (timestamp) and var_value (float). My query looks like this: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; So I want to select the last value from a determinated ID (is_signal). This query runs FOREVER, while if I delete limit 1 it runs instantly did you ANALYZE your tables? Can you send EXPLAIN ANALYZE result of both queries? Regards Pavel Stehule Any help? Regards. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] limit order by performance issue
On 10/15/2012 12:44 PM, Pedro Jiménez wrote: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; Well, we'd have to see an EXPLAIN plan to really know what's going on here, but it often boils down to the planner being overly optimistic when low limits are specified. I bet you have an index on time_stamp, don't you? In that case, the planner would reverse index-scan that index, estimating that the chances of it finding ID 29660 are less expensive than fetching all of the rows that match the ID directly, and throwing away all but 1 row. Remember, it would have to read all of those values to know which is the most recent. You can fix this a couple of ways: 1. Put a two-column index on these values: CREATE INDEX idx_ordered_signal ON ism_floatvalues (id_signal, time_stamp DESC); Which turns any request for that particular combo into a single index fetch. 2. You can trick the planner by introducing an optimization fence: SELECT var_value FROM ( SELECT var_value, time_stamp FROM ism_floatvalues WHERE id_signal = 29660 OFFSET 0 ) ORDER BY time_stamp DESC LIMIT 1; Quite a few people will probably grouse at me for giving you that as an option, but it does work better than LIMIT 1 more often than it probably should. -- Shaun Thomas OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604 312-444-8534 stho...@optionshouse.com __ See http://www.peak6.com/email_disclaimer/ for terms and conditions related to this email -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] limit order by performance issue
Put an index on time_stamp (I assume there is one on id_signal already) On 10/15/2012 12:44 PM, Pedro Jiménez wrote: Hello, I'm trying to do a simple SQL query over Postgresl 9.0 running on Ubuntu. I have a large table (over 100 million records) with three fields, id_signal (bigint), time_stamp (timestamp) and var_value (float). My query looks like this: select var_value from ism_floatvalues where id_signal = 29660 order by time_stamp desc limit 1; So I want to select the last value from a determinated ID (is_signal). This query runs FOREVER, while if I delete limit 1 it runs instantly Any help? Regards. -- -- Karl Denninger /The Market Ticker ®/ http://market-ticker.org Cuda Systems LLC
Re: [PERFORM] limit order by performance issue
On Tue, Oct 16, 2012 at 10:47 PM, Karl Denninger k...@denninger.net wrote: Put an index on time_stamp (I assume there is one on id_signal already) Well the optimal index for this particular query would include both columns: (id_signal, time_stamp) -- in this order. Additionally, if you want to take advantage of the index-only scans feature, add the SELECTed column too: (id_signal, time_stamp, var_value) Regards, Marti -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
