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>Chapter 49. Genetic Query Optimizer</TD
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><H1
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><A
NAME="GEQO-PG-INTRO"
>49.3. Genetic Query Optimization (<ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
>) in PostgreSQL</A
></H1
><P
> The <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
> module approaches the query
optimization problem as though it were the well-known traveling salesman
problem (<ACRONYM
CLASS="ACRONYM"
>TSP</ACRONYM
>).
Possible query plans are encoded as integer strings. Each string
represents the join order from one relation of the query to the next.
For example, the join tree
</P><PRE
CLASS="LITERALLAYOUT"
> /\
/\ 2
/\ 3
4 1</PRE
><P>
is encoded by the integer string '4-1-3-2',
which means, first join relation '4' and '1', then '3', and
then '2', where 1, 2, 3, 4 are relation IDs within the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> optimizer.
</P
><P
> Specific characteristics of the <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
>
implementation in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
are:
<P
></P
></P><UL
COMPACT="COMPACT"
><LI
STYLE="list-style-type: disc"
><P
> Usage of a <I
CLASS="FIRSTTERM"
>steady state</I
> <ACRONYM
CLASS="ACRONYM"
>GA</ACRONYM
> (replacement of the least fit
individuals in a population, not whole-generational replacement)
allows fast convergence towards improved query plans. This is
essential for query handling with reasonable time;
</P
></LI
><LI
STYLE="list-style-type: disc"
><P
> Usage of <I
CLASS="FIRSTTERM"
>edge recombination crossover</I
>
which is especially suited to keep edge losses low for the
solution of the <ACRONYM
CLASS="ACRONYM"
>TSP</ACRONYM
> by means of a
<ACRONYM
CLASS="ACRONYM"
>GA</ACRONYM
>;
</P
></LI
><LI
STYLE="list-style-type: disc"
><P
> Mutation as genetic operator is deprecated so that no repair
mechanisms are needed to generate legal <ACRONYM
CLASS="ACRONYM"
>TSP</ACRONYM
> tours.
</P
></LI
></UL
><P>
</P
><P
> Parts of the <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
> module are adapted from D. Whitley's
Genitor algorithm.
</P
><P
> The <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
> module allows
the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> query optimizer to
support large join queries effectively through
non-exhaustive search.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN75655"
>49.3.1. Generating Possible Plans with <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
></A
></H2
><P
> The <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
> planning process uses the standard planner
code to generate plans for scans of individual relations. Then join
plans are developed using the genetic approach. As shown above, each
candidate join plan is represented by a sequence in which to join
the base relations. In the initial stage, the <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
>
code simply generates some possible join sequences at random. For each
join sequence considered, the standard planner code is invoked to
estimate the cost of performing the query using that join sequence.
(For each step of the join sequence, all three possible join strategies
are considered; and all the initially-determined relation scan plans
are available. The estimated cost is the cheapest of these
possibilities.) Join sequences with lower estimated cost are considered
<SPAN
CLASS="QUOTE"
>"more fit"</SPAN
> than those with higher cost. The genetic algorithm
discards the least fit candidates. Then new candidates are generated
by combining genes of more-fit candidates — that is, by using
randomly-chosen portions of known low-cost join sequences to create
new sequences for consideration. This process is repeated until a
preset number of join sequences have been considered; then the best
one found at any time during the search is used to generate the finished
plan.
</P
><P
> This process is inherently nondeterministic, because of the randomized
choices made during both the initial population selection and subsequent
<SPAN
CLASS="QUOTE"
>"mutation"</SPAN
> of the best candidates. Hence different plans may
be selected from one run to the next, resulting in varying run time
and varying output row order.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="GEQO-FUTURE"
>49.3.2. Future Implementation Tasks for
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> <ACRONYM
CLASS="ACRONYM"
>GEQO</ACRONYM
></A
></H2
><P
> Work is still needed to improve the genetic algorithm parameter
settings.
In file <TT
CLASS="FILENAME"
>src/backend/optimizer/geqo/geqo_main.c</TT
>,
routines
<CODE
CLASS="FUNCTION"
>gimme_pool_size</CODE
> and <CODE
CLASS="FUNCTION"
>gimme_number_generations</CODE
>,
we have to find a compromise for the parameter settings
to satisfy two competing demands:
<P
></P
></P><UL
COMPACT="COMPACT"
><LI
><P
> Optimality of the query plan
</P
></LI
><LI
><P
> Computing time
</P
></LI
></UL
><P>
</P
><P
> In the current implementation, the fitness of each candidate join
sequence is estimated by running the standard planner's join selection
and cost estimation code from scratch. To the extent that different
candidates use similar sub-sequences of joins, a great deal of work
will be repeated. This could be made significantly faster by retaining
cost estimates for sub-joins. The problem is to avoid expending
unreasonable amounts of memory on retaining that state.
</P
><P
> At a more basic level, it is not clear that solving query optimization
with a GA algorithm designed for TSP is appropriate. In the TSP case,
the cost associated with any substring (partial tour) is independent
of the rest of the tour, but this is certainly not true for query
optimization. Thus it is questionable whether edge recombination
crossover is the most effective mutation procedure.
</P
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