On 12/30/2016 12:46 AM, David G. Johnston wrote:
On Thu, Dec 29, 2016 at 4:38 PM, Tomas Vondra
<tomas.vondra@xxxxxxxxxxxxxxx <mailto:tomas.vondra@2ndquadrant.com >>wrote:
On 12/30/2016 12:33 AM, David G. Johnston wrote:
On Thu, Dec 29, 2016 at 4:21 PM, Job <Job@xxxxxxxxxxxxxxxxxxxx
<mailto:Job@colliniconsulting.it >
<mailto:Job@colliniconsulting.it
<mailto:Job@colliniconsulting.it >>>wrote:
Hello,
in Postgresql 9.6 we have a query running on a very large table
based, in some cases, on a like statement:
... WHERE FIELD='CODE' OR FIELD LIKE 'CODE_%'
Which type of index can i create to speed to the search when the
"like" case happens?
GIST
https://www.postgresql.org/docs/9.6/static/pgtrgm.html
<https://www.postgresql.org/docs/9.6/static/pgtrgm.html >
https://www.postgresql.org/docs/9.6/static/btree-gist.html
<https://www.postgresql.org/docs/9.6/static/btree-gist.html >
For prefix queries, it's also possible to use simple btree index
with varchar_pattern_ops.
https://www.postgresql.org/docs/9.6/static/indexes-opclass. html
<https://www.postgresql.org/docs/9.6/static/indexes-opclass. >html
Even knowing that this feature exists I don't know that I could have
found it within a reasonable amount of time in its present location. A
few cross-references from elsewhere (probably at least the functions
part of the documentation) would make learning about the capability a
lot easier.
Well, it's referenced right from the "Indexes" part of the documentation (right at the beginning of "Index Types"):
https://www.postgresql.org/docs/9.6/static/indexes.html
While I may have an understanding of what operator classes and families are when I am in my SQL thinking mode those terms don't really come to mind. Maybe part of the problem is that SQL doesn't have indexes and so my formal education never covered them. I learned how to use "CREATE INDEX" to meet most common needs but the fact that I'm getting a b-tree family index is well hidden.
While I'm all for learning the theory a more prescriptive approach (do this to get an index that will allow prefix LIKEs to use it - see this section for detail) to the topic would be welcome. Tell the user how to use an index when they are learning about the feature that they care about - LIKE - not require them to learn all about indexes and later realize/remember that one particular incantation will solve the LIKE problem.
David J.
