Subject: Is there a way to translate pg_amop.amopstrategy into a description?

From: Morris de Oryx <[email-masked]>
Date: Thu, 22 Aug 2024 17:34:39 -0500

I'm digging into GiST indexes again, and ran into a helpful script here:

https://medium.com/postgres-professional/indexes-in-postgresql-5-gist-86e19781b5db

(This piece has shown up in many places in various versions.) I've adapted
the search a little, as I'd like to make it easier to explore available
index ops:

 SELECT amop.amopopr::regoperator                       AS operator,
        iif(amop.amoppurpose = 's', 'search','order')   AS purpose,
        amop.amopstrategy                               AS
stratgey_number -- I'd like to translate this into a description

   FROM pg_opclass opc,
        pg_opfamily opf,
        pg_am am,
        pg_amop amop

  WHERE opc.opcname        = 'gist_trgm_ops'
    AND am.amname          = 'gist'
    AND opf.oid            = opc.opcfamily
    AND am.oid             = opf.opfmethod
    AND amop.amopfamily    = opc.opcfamily
    AND amop.amoplefttype  = opc.opcintype;

+------------------+---------+-----------------+
| operator         | purpose | stratgey_number |
+------------------+---------+-----------------+
| %(text,text)     | search  | 1               |
| <->(text,text)   | order   | 2               |
| ~~(text,text)    | search  | 3               |
| ~~*(text,text)   | search  | 4               |
| ~(text,text)     | search  | 5               |
| ~*(text,text)    | search  | 6               |
| %>(text,text)    | search  | 7               |
| <->>(text,text)  | order   | 8               |
| %>>(text,text)   | search  | 9               |
| <->>>(text,text) | order   | 10              |
| =(text,text)     | search  | 11              |
+------------------+---------+-----------------+

What I'm hoping for is a function like
get_opt_class_strategy_description(optclass, straregy_number)  I've
looked at the source a bit, and it seems that there is no such
function, and that it might well be difficult to implement. The
strategy numbers are, as far as I can see, local to the specific
opt_class, which has no requirement to label them in any particular
way.

Does anyone know if I'm missing something?

Along the way, I did find that you can often look things up by hand in
the source for specific tools, or review a lot of the strategies in
one place:

https://github.com/postgres/postgres/blob/edcb71258504ed22abba8cc7181d2bab3762e757/src/include/catalog/pg_amop.dat#L82

It's easier to use the docs at that point.

No lives hang in the balance here, but I'm hoping to learn something.

Thanks for any help or clarification.

---

From: Tom Lane <[email-masked]>
Date: Thu, 22 Aug 2024 18:42:15 -0400

> opt_class, which has no requirement to label them in any particular

That's correct.  For btree and hash, the meanings of the strategy
numbers are determined by the index AM; but for (IIRC) all of our
other index AMs they're determined by the individual opclass.  So
anything like this would have to be implemented by dedicated code
in each opclass.  Perhaps that's worth doing, but it'd be a fair
amount of work.

			regards, tom lane