Re: [HACKERS] Speedups

Vadim wrote:
>
> ocie@paracel.com wrote:
> >
> > > > Not sure ofhand, but it would be useful for JDBC's PreparedStatement and
> > > > CallableStatement classes
> > >
> > > We can implement it very easy, and fast. Execution plan may be reused
> > > many times. Is this feature in standard ?
> > > What is proposed syntax if not ?
> >
> > I do see a couple sticky points:
> >
> > We would need some information about which variables are to be
> > substituted into this query plan, but this should be fairly
> > straightforward.
>
> Parser, Planner/Optimizer and Executor are able to handle parameters!
> No problems with this.

    Nice discussion - especially when looking at what I initially
    posted.

    I assume you think about using SPI's saved plan  feature  for
    it. Right?

>
> > Some querys may not respond well to this, for example, if a table had
> > an index on an integer field f1, this would probably be the best way
> > to satisfy a select where f1<10.  But if this were in a query as f1<x,
> > then a sufficiently high value of x might make this not such a good
> > way to run the query.  I haven't looked into this, but I would assume
> > that the optimizer relies on the specific values in such cases.
>                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Unfortunately, no!
> We have to add this feature of 'course.
> I don't know how we could deal with pre-compiled plans after this :(
> May be, we could prepare/store not single plan, but some number of
> possible plans.

    That's  something  I  thought  about  when  I  used  the  SPI
    functions when I built PL/Tcl. Since the saved plan  will  be
    executed via SPI_execp(), we could change some details there.
    Currently SPI_prepare() and SPI_saveplan()  return  the  plan
    itself.  But  they  could also return a little control struct
    that contains the plan and other information. Since  I  don't
    expect  someone  uses  these  plans  for  something else than
    calling SPI_execp(), it wouldn't break anything.

    SPI_execp()  can  do  some  timing  calculations.  For   each
    execution  of  a plan it collects the runtime in microseconds
    (gettimeofday()). After the 5th or 10th call,  it  builds  an
    average  and  remembers that permanently.  For all subsequent
    calls it calculates the average time of the last 10 calls and
    if that gets much higher than the initial average it wouldn't
    hurt to silently prepare  and  save  the  plan  again.  Using
    averages   lowers   the   problem  that  differences  in  the
    parameters can cause the runtime differences.

    Another possible reason for the runtime  differences  is  the
    overall  workload  of  the  server.  This  could be very high
    during the initial average calculation. So I think  it  could
    be  smart  to rebuild the plan after e.g. 1000 calls ignoring
    any runtimes.

>
> > We need to be able to handle changes to the structures and contents of
> > the tables.  If the query plan is built and we add 10000 rows to a
> > table it references, the query should probably be recompiled.  We
> > could probably do this at vacuum time.  There is also a small chance
> > that a table or index that the query plan was using is dropped.  We
> > could automatically rebuild the query if the table was created after
> > the query was compiled.
>
> We could mark stored plans as durty in such cases to force re-compiling
> when an application tries to use this plan.

    Yep. SPI must remember all  prepared  and  saved  plans  (and
    forget  about  only prepared ones at transaction end). Things
    like dropping an index or modifying a table  structure  cause
    invalidations in the relcache, syscache and catcache (even if
    another backend did it in some cases).  I think  it  must  be
    possible  to  tell SPI from there that something happened and
    which relations are affected. If a plans rangetable  contains
    the affected relation, the plan is marked durty.

    Things  like  functions,  operators  and  aggregates are also
    objects that might change (drop/recreate function -> funcnode
    in plan get's unusable).

    I  think  the  best  would  be  that SPI_prepare() set's up a
    collection of Oid's  that  cause  plan  invalidation  in  the
    control  structure.   These  are  the  Oid's  of  ALL objects
    (relations, indices, functions etc.)  used in the plan.  Then
    a  call  to  SPI_invalidate(Oid)  from the cache invalidation
    handlers doesn't have to walk through the plan itself.


Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#======================================== jwieck@debis.com (Jan Wieck) #