Terrible performance on wide selects
От | Steve Crawford |
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Тема | Terrible performance on wide selects |
Дата | |
Msg-id | 20030117193726.1CFCB103E5@polaris.pinpointresearch.com обсуждение исходный текст |
Ответы |
Re: Terrible performance on wide selects
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Список | pgsql-general |
I have a table which is rather wide (~800 columns) and consists of a few columns of identifying data (run time, channel and such) and up to several hundred columns of collected data (no, normalization does not suggest putting collected data in another table - collected item 1 always corresponds to collected item 1 but is completely different than item 3). My test table is very short (62 rows) but in production would grow by several thousand rows per day. Unfortunately if my test data is correct, performance on wide selects is so bad that it will render the system unusable. Here's the test. I have created two versions of the table - one stores the collected data in an array of text and the other stores the data in individual columns, no joins, no indexes. Times are averages of many runs - the times varied very little and the data is small enough that I'm sure it was served from RAM. Postgres CPU utilization observed on the longer runs was 98-99%. Changing the output format didn't seem to change things significantly. Times for selecting all the columns in the table: select * from columnversion; 8,000 ms select * from arrayversion; 110 ms select * from arraytocolumnview (data in the array version but converted to columns in the view) 10,000 ms Times to select a single column in a table: select runstarttime from columversion; 32 ms select runstarttime from arrayversion; 6 ms So the question is, does it seem reasonable that a query on fundamentally identical data should take 70-90 times as long when displayed as individual columns vs. when output as a raw array and, more imporantly, what can I do to get acceptable performance on this query? Cheers, Steve
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