Re: pg_trgm version 1.2

On Tue, Jun 30, 2015 at 2:46 AM, Alexander Korotkov <a.korotkov@postgrespro.ru> wrote:

On Sun, Jun 28, 2015 at 1:17 AM, Jeff Janes <jeff.janes@gmail.com> wrote:

This patch implements version 1.2 of contrib module pg_trgm.

This supports the triconsistent function, introduced in version 9.4 of the server, to make it faster to implement indexed queries where some keys are common and some are rare.

Thank you for the patch! Lack of pg_trgm triconsistent support was significant miss after "fast scan" implementation.

 

I've included the paths to both upgrade and downgrade between 1.1 and 1.2, although after doing so you must close and restart the session before you can be sure the change has taken effect. There is no change to the on-disk index structure

pg_trgm--1.1.sql andpg_trgm--1.1--1.2.sql are useful for debug, but do you expect them in final commit? As I can see in other contribs we have only last version and upgrade scripts.

I had thought that pg_trgm--1.1.sql was needed for pg_upgrade to work, but I see that that is not the case.

I did see another downgrade path for different module, but on closer inspection it was one that I wrote while trying to analyze that module, and then never removed.  I have no objection to removing pg_trgm--1.2--1.1.sql before the commit, but I don't see what we gain by doing so.  If a downgrade is feasible and has been tested, why not include it?

You could get the same benefit just by increasing MAX_MAYBE_ENTRIES (in core) from 4 to some higher value (which it probably should be anyway, but there will always be a case where it needs to be higher than you can afford it to be, so a real solution is needed).

 

Actually, it depends on how long it takes to calculate consistent function. The cheaper consistent function is, the higher MAX_MAYBE_ENTRIES could be. Since all functions in PostgreSQL may define its cost, GIN could calculate MAX_MAYBE_ENTRIES from the cost of consistent function.

The consistent function gets called on every candidate tuple, so if it is expensive then it is also all the more worthwhile to reduce the set of candidate tuples.  Perhaps MAX_MAYBE_ENTRIES could be calculated from the log of the maximum of the predictNumberResult entries? Anyway, a subject for a different day....

There may also be some gains in the similarity and regex cases, but I didn't really analyze those for performance.

 

I've thought about how to document this change.  Looking to other example of other contrib modules with multiple versions, I decided that we don't document them, other than in the release notes.

The same patch applies to 9.4 code with a minor conflict in the Makefile, and gives benefits there as well.

Some other notes about the patch:

* You allocate boolcheck array and don't use it. 

That was a bug (at least notionally).  trigramsMatchGraph was supposed to be getting boolcheck, not check, passed in to it.

It may not have been a bug in practise, because GIN_MAYBE and GIN_TRUE both test as true when cast to booleans. But it seems wrong to rely on that.  Or was it intended to work this way?

I'm surprised the compiler suite doesn't emit some kind of warning on that.

 

* Check coding style and formatting, in particular "check[i]==GIN_TRUE" should be "check[i] == GIN_TRUE".

Sorry about that, fixed.  I also changed it in other places to check[i] != GIN_FALSE, rather than checking against both GIN_TRUE and GIN_MAYBE.  At first I was concerned we might decide to add a 4th option to the type which would render  != GIN_FALSE the wrong way to test for it.  But since it is called GinTernaryValue, I think we wouldn't add a fourth thing to it.  But perhaps the more verbose form is clearer to some people.

* I think some comments needed in gin_trgm_triconsistent() about trigramsMatchGraph(). gin_trgm_triconsistent() may use trigramsMatchGraph() that way because trigramsMatchGraph() implements monotonous boolean function.

I have a function-level comment that in all cases, GIN_TRUE is at least as favorable to inclusion of a tuple as GIN_MAYBE.  Should I reiterate that comment before trigramsMatchGraph() as well?  Calling it a monotonic boolean function is precise and concise, but probably less understandable to people reading the code.  At least, if I saw that, I would need to go do some reading before I knew what it meant.