Re: pg_stat_statements and "IN" conditions

Hi,

On 2/9/23 16:02, Dmitry Dolgov wrote:
>> Unfortunately, rebase is needed again due to recent changes in queryjumblefuncs (
9ba37b2cb6a174b37fc51d0649ef73e56eae27fc)
 
I reviewed the last patch applied to some commit from Feb. 4th.
>> It seems a little strange to me that with const_merge_threshold = 1, such a test case gives the same result as with
const_merge_threshold= 2
 
>>
>> select pg_stat_statements_reset();
>> set const_merge_threshold to 1;
>> select * from test where i in (1,2,3);
>> select * from test where i in (1,2);
>> select * from test where i in (1);
>> select query, calls from pg_stat_statements order by query;
>>
>>                  query                | calls
>> -------------------------------------+-------
>>   select * from test where i in (...) |     2
>>   select * from test where i in ($1)  |     1
>>
>> Probably const_merge_threshold = 1 should produce only "i in (...)"?
> Well, it's not intentional, probably I need to be more careful with
> off-by-one. Although I agree to a certain extent with Peter questioning

Please add tests for all the corner cases. At least for (1) IN only 
contains a single element and (2) const_merge_threshold = 1.

Beyond that:

- There's a comment about find_const_walker(). I cannot find that 
function anywhere. What am I missing?

- What about renaming IsConstList() to IsMergableConstList().

- Don't you intend to use the NUMERIC data column in SELECT * FROM 
test_merge_numeric WHERE id IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10)? 
Otherwise, the test is identical to previous test cases and you're not 
checking for what happens with NUMERICs which are wrapped in FuncExpr 
because of the implicit coercion.

- Don't we want to extend IsConstList() to allow a list of all 
implicitly coerced constants? It's inconsistent that otherwise e.g. 
NUMERICs don't work.

- Typo in /* The firsts merged constant */ (first not firsts)

- Prepared statements are not supported as they contain INs with Param 
instead of Const nodes. While less likely, I've seen applications that 
use prepared statements in conjunction with queries generated through a 
UI which ended up with tons of prepared queries with different number of 
elements in the IN clause. Not necessarily something that must go into 
this patch but maybe worth thinking about.

- The setting name const_merge_threshold is not very telling without 
knowing the context. While being a little longer, what about 
jumble_const_merge_threshold or queryid_const_merge_threshold or similar?

- Why do we actually only want to merge constants? Why don't we ignore 
the type of element in the IN and merge whatever is there? Is this 
because the original jumbling logic as of now only has support for 
constants?

- Ideally we would even remove duplicates. That would even improve 
cardinality estimation but I guess we don't want to spend the cycles on 
doing so in the planner?

- Why did you change palloc() to palloc0() for clocations array? The 
length is initialized to 0 and FWICS RecordConstLocation() initializes 
all members. Seems to me like we don't have to spend these cycles.

- Can't the logic at the end of IsConstList() not be simplified to:

         foreach(temp, elements)
             if (!IsA(lfirst(temp), Const))
                 return false;

         // All elements are of type Const
         *firstConst = linitial_node(Const, elements);
         *lastConst = llast_node(Const, elements);
         return true;

-- 
David Geier
(ServiceNow)