Re: [HACKERS] path toward faster partition pruning

David Rowley wrote:

> 2. I guess this will be removed before commit.
> 
> +#if 0
> <large section of comments>
> +#endif

Yeah, there is one sentence there I didn't quite understand and would
like to add it to the rewritten version of the comment before I remove
the whole ifdeffed-out comment.

 *    PARTCLAUSE_MATCH_STEPS: *clause_steps set to list of "partition pruning
 *    step(s)" generated for the clause due to it being a BoolExpr or a
 *    ScalarArrayOpExpr that's turned into one

Exactly what does "ScalarArrayOpExpr that's turned into one" means?
Does it mean we turn SAOP into BoolExpr?

(Yes, I know "#if 0" inside a comment doesn't do anything.  It's only
documentation for myself.)

If you look at the rest of the rewritten comment, you'll notice some
things probably need more explaining.  Wording suggestions welcome.

> 3. This comment seems like a strange thing to write just before
> testing if the clause matches the partition key.
> 
> + /* Clause does not match this partition key. */
> + if (equal(leftop, partkey))
> + *rightop = not_clause((Node *) clause)
> + ? (Expr *) makeBoolConst(false, false)
> + : (Expr *) makeBoolConst(true, false);

Yeah.  Looking at this function, I noticed it tests for BooleanTest, and
falls back to checking "not_clause" and a few equals.  Does it make
sense if the clause is a SAOP?  I added this assert:
    Assert(IsA(clause, BooleanTest) ||
           IsA(clause, BoolExpr) ||
           IsA(clause, RelabelType));

and it failed:
#3  0x0000556cf04505db in match_boolean_partition_clause (partopfamily=424, 
    clause=0x556cf1041670, partkey=0x556cf1042218, rightop=0x7ffe520ec068)
    at /pgsql/source/master/src/backend/optimizer/util/partprune.c:2159
2159        Assert(IsA(clause, BooleanTest) ||
(gdb) print *clause
$1 = {type = T_ScalarArrayOpExpr}

I'm not sure whether or not this function can trust that what's incoming
must absolutely be only those node types.

> 4. Comment needs removed.
> 
> + * has_default_part - Whether the table has a default partition

Done.

> The only other thing I noted on this pass is that we could get rid of:
> 
> + /* go check the next clause. */
> + if (unsupported_clause)
> + break;
> 
> and just "continue" instead of "break" in all cases apart from case
> PARTCLAUSE_UNSUPPORTED:
> 
> it would save a few lines and a single condition. What's there works,
> but thought this might be better...

Makes sense -- looking.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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