Thread: seq scan only when function not in subquery (bug?)
sas=> explain update user set usrid = 'aaaaaaaa' where usrseqid=usrseqid('zlb'); NOTICE: QUERY PLAN: Seq Scan on user (cost=344.07 size=658 width=154) EXPLAIN sas=> explain update user set usrid = 'aaaaaaaa' where usrseqid=(select usrseqid('zlb')); NOTICE: QUERY PLAN: Index Scan on user (cost=1.05 size=1 width=154) InitPlan -> Result (cost=0.00 size=0 width=0) EXPLAIN sas=> as you can see, it uses the index when the RHS of the comparison in the where clause is a subquery, but a sequential scan when it isn't. is this a bug?
Brett McCormick wrote: > > sas=> explain update user set usrid = 'aaaaaaaa' > where usrseqid=usrseqid('zlb'); > > NOTICE: QUERY PLAN: > > Seq Scan on user (cost=344.07 size=658 width=154) ... > as you can see, it uses the index when the RHS of the comparison > in the where clause is a subquery, but a sequential scan when it > isn't. is this a bug? Yes, and very old :) This is from my recent posting: --- Another issue - handling of functions with constant args in queries - for query select * from T where A = upper ('bbb') function upper ('bbb') will be executed for each tuple in T! More of that - if there is index on T(A) then this index will not be used for this query! Obviously, upper ('bbb') should be executed (by Executor, not parser/planner) once: new Param type (PARAM_EXEC) implemented for subselects could help here too... --- Actually, this is easy to fix... Vadim
On Tue, 16 June 1998, at 10:32:39, Vadim Mikheev wrote: > Another issue - handling of functions with constant args > in queries - for query > > select * from T where A = upper ('bbb') > > function upper ('bbb') will be executed for each tuple in T! > More of that - if there is index on T(A) then this index will > not be used for this query! > Obviously, upper ('bbb') should be executed (by Executor, not > parser/planner) once: new Param type (PARAM_EXEC) implemented > for subselects could help here too... > --- > > Actually, this is easy to fix... I was going to reply to this but never did -- how do you tell if it needs to be executed once per query or once per tuple? What if you wanted to call a function which returned a different value for each tuple, like random()?
Re: [HACKERS] seq scan only when function not in subquery (bug?)
From
dg@illustra.com (David Gould)
Date:
> On Tue, 16 June 1998, at 10:32:39, Vadim Mikheev wrote: > > > Another issue - handling of functions with constant args > > in queries - for query > > > > select * from T where A = upper ('bbb') > > > > function upper ('bbb') will be executed for each tuple in T! > > More of that - if there is index on T(A) then this index will > > not be used for this query! > > Obviously, upper ('bbb') should be executed (by Executor, not > > parser/planner) once: new Param type (PARAM_EXEC) implemented > > for subselects could help here too... > > --- > > > > Actually, this is easy to fix... > > I was going to reply to this but never did -- how do you tell if it > needs to be executed once per query or once per tuple? What if you > wanted to call a function which returned a different value for each > tuple, like random()? To make this work, you need an attribute in the functions table (and internal info about the function) that tells if the function is "variant" or not. A variant function can return different results with the same arguments eg random(), or has side effects. A non variant function returns the same result for the same arguments and has no side-effects. If you have a non-variant function, then the easy way to optimize it is to memoize the arguments and result of the last time you called it. Then the next time you want to call it, check if the arguments are the same and if so, merely return the previously saved result instead of calling the function. Example: create function city_from_zipcode(integer) returns varchar not variant; select name, street, city_from_zipcode(zipcode), zipcode from (select * from customers order by zipcode); If customers was sorted by zipcode, this would only call city_from_zipcode() each time the zipcode changed instead of for each row. It would also cover the case of "function('constant'); -dg David Gould dg@illustra.com 510.628.3783 or 510.305.9468 Informix Software 300 Lakeside Drive Oakland, CA 94612 - A child of five could understand this! Fetch me a child of five.