2016-01-28 16:03 GMT-02:00 Joshua D. Drake <jd@xxxxxxxxxxxxxxxxx>:
On 01/28/2016 09:41 AM, Melvin Davidson wrote:
So, IOW, and the answer to my question is yes, it should be insured that
all pages involved are physically adjacent (by design or by pre-sort)
before creating a BRIN on them.
Further to the point, it is self defeating to have more than one BRIN
index on the table if the columns involved would have mutually
non-adjacent pages.
Therefore, it actually would be good to state that in the documentation,
even it were just a comment.
BRIN indexes are best used on INSERT only tables with a sequence of numbers as a PK or indexed column that will be queried against. At least as I understand it.
JD
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"Further to the point, it is self defeating to have more than one BRIN index on the table if the columns involved would have mutually non-adjacent pages."
Not really, if both columns are ordered, BRIN will work
It is = "BRIN is designed for handling very large tables in which certain columns have some natural correlation with their physical location within the table"
Also, I did some tests and here are the results I got:
Query with no index = completion time 43s
Same Query with BRIN = completion time 14s / index size 0,5 MB
Same Query without BRIN and with BTREE = completion time 10s / index size 5.000,00 MB
As you can see, BRIN can save 99% of disk space for just a slightly worse performance.
It seems like a huge improvement, given that your data fits BRIN's use case.
