path: root/doc/src/sgml/perform.sgml

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 <chapter id="performance-tips">
  <title>Performance Tips</title>

  <para>
   Query performance can be affected by many things. Some of these can 
   be manipulated by the user, while others are fundamental to the underlying
   design of the system.  This chapter provides some hints about understanding
   and tuning <productname>Postgres</productname> performance.
  </para>

  <sect1 id="using-explain">
   <title>Using <command>EXPLAIN</command></title>

   <note>
    <title>Author</title>
    <para>
     Written by Tom Lane, from e-mail dated 2000-03-27.
    </para>
   </note>

   <para>
    <productname>Postgres</productname> devises a <firstterm>query
    plan</firstterm> for each query it is given.  Choosing the right
    plan to match the query structure and the properties of the data
    is absolutely critical for good performance.  You can use the
    <command>EXPLAIN</command> command to see what query plan the system
    creates for any query.  Unfortunately,
    plan-reading is an art that deserves a tutorial, and I haven't
    had time to write one.  Here is some quick &amp; dirty explanation.
   </para>

   <para>
    The numbers that are currently quoted by EXPLAIN are:

    <itemizedlist>
     <listitem>
      <para>
       Estimated start-up cost (time expended before output scan can start,
       e.g., time to do the sorting in a SORT node).
      </para>
     </listitem>

     <listitem>
      <para>
       Estimated total cost (if all tuples are retrieved, which they may not
       be --- a query with a LIMIT will stop short of paying the total cost,
       for example).
      </para>
     </listitem>

     <listitem>
      <para>
       Estimated number of rows output by this plan node (again, without
       regard for any LIMIT).
      </para>
     </listitem>

     <listitem>
      <para>
       Estimated average width (in bytes) of rows output by this plan
       node.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The costs are measured in units of disk page fetches.  (CPU effort
    estimates are converted into disk-page units using some
    fairly arbitrary fudge-factors. If you want to experiment with these
    factors, see the list of run-time configuration parameters in the
    <citetitle>Administrator's Guide</citetitle>.)
   </para>

   <para>
    It's important to note that the cost of an upper-level node includes
    the cost of all its child nodes.  It's also important to realize that
    the cost only reflects things that the planner/optimizer cares about.
    In particular, the cost does not consider the time spent transmitting
    result tuples to the frontend --- which could be a pretty dominant
    factor in the true elapsed time, but the planner ignores it because
    it cannot change it by altering the plan.  (Every correct plan will
    output the same tuple set, we trust.)
   </para>

   <para>
    Rows output is a little tricky because it is <emphasis>not</emphasis> the
    number of rows
    processed/scanned by the query --- it is usually less, reflecting the
    estimated selectivity of any WHERE-clause constraints that are being
    applied at this node.  Ideally the top-level rows estimate will
    approximate the number of rows actually returned, updated, or deleted
    by the query (again, without considering the effects of LIMIT).
   </para>

   <para>
    Here are some examples (using the regress test database after a
    vacuum analyze, and almost-7.0 sources):

    <programlisting>
regression=# explain select * from tenk1;
NOTICE:  QUERY PLAN:

Seq Scan on tenk1  (cost=0.00..333.00 rows=10000 width=148)
    </programlisting>
   </para>

   <para>
    This is about as straightforward as it gets.  If you do

    <programlisting>
select * from pg_class where relname = 'tenk1';
    </programlisting>

    you'll find out that tenk1 has 233 disk
    pages and 10000 tuples.  So the cost is estimated at 233 block
    reads, defined as 1.0 apiece, plus 10000 * cpu_tuple_cost which is
    currently 0.01 (try <command>show cpu_tuple_cost</command>).
   </para>

   <para>
    Now let's modify the query to add a qualification clause:

    <programlisting>
regression=# explain select * from tenk1 where unique1 &lt; 1000;
NOTICE:  QUERY PLAN:

Seq Scan on tenk1  (cost=0.00..358.00 rows=1000 width=148)
    </programlisting>

    The estimate of output rows has gone down because of the WHERE clause.
    (This estimate is uncannily accurate because tenk1 is a particularly
    simple case --- the unique1 column has 10000 distinct values ranging
    from 0 to 9999, so the estimator's linear interpolation between min and
    max column values is dead-on.)  However, the scan will still have to
    visit all 10000 rows, so the cost hasn't decreased; in fact it has gone
    up a bit to reflect the extra CPU time spent checking the WHERE
    condition.
   </para>

   <para>
    Modify the query to restrict the qualification even more:

    <programlisting>
regression=# explain select * from tenk1 where unique1 &lt; 100;
NOTICE:  QUERY PLAN:

Index Scan using tenk1_unique1 on tenk1  (cost=0.00..89.35 rows=100 width=148)
    </programlisting>

    and you will see that if we make the WHERE condition selective
    enough, the planner will
    eventually decide that an indexscan is cheaper than a sequential scan.
    This plan will only have to visit 100 tuples because of the index,
    so it wins despite the fact that each individual fetch is expensive.
   </para>

   <para>
    Add another condition to the qualification:

    <programlisting>
regression=# explain select * from tenk1 where unique1 &lt; 100 and
regression-# stringu1 = 'xxx';
NOTICE:  QUERY PLAN:

Index Scan using tenk1_unique1 on tenk1  (cost=0.00..89.60 rows=1 width=148)
    </programlisting>

    The added clause "stringu1 = 'xxx'" reduces the output-rows estimate,
    but not the cost because we still have to visit the same set of tuples.
   </para>

   <para>
    Let's try joining two tables, using the fields we have been discussing:

    <programlisting>
regression=# explain select * from tenk1 t1, tenk2 t2 where t1.unique1 &lt; 100
regression-# and t1.unique2 = t2.unique2;
NOTICE:  QUERY PLAN:

Nested Loop  (cost=0.00..144.07 rows=100 width=296)
  -&gt;  Index Scan using tenk1_unique1 on tenk1 t1
             (cost=0.00..89.35 rows=100 width=148)
  -&gt;  Index Scan using tenk2_unique2 on tenk2 t2
             (cost=0.00..0.53 rows=1 width=148)
    </programlisting>
   </para>

   <para>
    In this nested-loop join, the outer scan is the same indexscan we had
    in the example before last, and so its cost and row count are the same
    because we are applying the "unique1 &lt; 100" WHERE clause at that node.
    The "t1.unique2 = t2.unique2" clause isn't relevant yet, so it doesn't
    affect the outer scan's row count.  For the inner scan, the
    current
    outer-scan tuple's unique2 value is plugged into the inner indexscan
    to produce an indexqual like
    "t2.unique2 = <replaceable>constant</replaceable>".  So we get the
     same inner-scan plan and costs that we'd get from, say, "explain select
     * from tenk2 where unique2 = 42".  The loop node's costs are then set
     on the basis of the outer scan's cost, plus one repetition of the
     inner scan for each outer tuple (100 * 0.53, here), plus a little CPU
     time for join processing.
   </para>

   <para>
    In this example the loop's output row count is the same as the product
    of the two scans' row counts, but that's not true in general, because
    in general you can have WHERE clauses that mention both relations and
    so can only be applied at the join point, not to either input scan.
    For example, if we added "WHERE ... AND t1.hundred &lt; t2.hundred",
    that'd decrease the output row count of the join node, but not change
    either input scan.
   </para>

   <para>
    One way to look at variant plans is to force the planner to disregard
    whatever strategy it thought was the winner, using the enable/disable
    flags for each plan type.  (This is a crude tool, but useful.  See
    also <xref linkend="explicit-joins">.)

    <programlisting>
regression=# set enable_nestloop = off;
SET VARIABLE
regression=# explain select * from tenk1 t1, tenk2 t2 where t1.unique1 < 100
regression-# and t1.unique2 = t2.unique2;
NOTICE:  QUERY PLAN:

Hash Join  (cost=89.60..574.10 rows=100 width=296)
  -&gt;  Seq Scan on tenk2 t2
               (cost=0.00..333.00 rows=10000 width=148)
  -&gt;  Hash  (cost=89.35..89.35 rows=100 width=148)
        -&gt;  Index Scan using tenk1_unique1 on tenk1 t1
               (cost=0.00..89.35 rows=100 width=148)
    </programlisting>

    This plan proposes to extract the 100 interesting rows of tenk1
    using ye same olde indexscan, stash them into an in-memory hash table,
    and then do a sequential scan of tenk2, probing into the hash table
    for possible matches of "t1.unique2 = t2.unique2" at each tenk2 tuple.
    The cost to read tenk1 and set up the hash table is entirely start-up
    cost for the hash join, since we won't get any tuples out until we can
    start reading tenk2.  The total time estimate for the join also
    includes a pretty hefty charge for CPU time to probe the hash table
    10000 times.  Note, however, that we are NOT charging 10000 times 89.35;
    the hash table setup is only done once in this plan type.
   </para>
  </sect1>

 <sect1 id="explicit-joins">
  <title>Controlling the Planner with Explicit JOINs</title>

  <para>
   Beginning with <productname>Postgres</productname> 7.1 it is possible
   to control the query planner to some extent by using explicit JOIN
   syntax.  To see why this matters, we first need some background.
  </para>

  <para>
   In a simple join query, such as
    <programlisting>
SELECT * FROM a,b,c WHERE a.id = b.id AND b.ref = c.id;
    </programlisting>
   the planner is free to join the given tables in any order.  For example,
   it could generate a query plan that joins A to B, using the WHERE clause
   a.id = b.id, and then joins C to this joined table, using the other
   WHERE clause.  Or it could join B to C and then join A to that result.
   Or it could join A to C and then join them with B --- but that would
   be inefficient, since the full Cartesian product of A and C would have
   to be formed, there being no applicable WHERE clause to allow optimization
   of the join.
   (All joins in the <productname>Postgres</productname> executor happen
   between two input tables, so it's necessary to build up the result in one
   or another of these fashions.)  The important point is that these different
   join possibilities give semantically equivalent results but may have hugely
   different execution costs.  Therefore, the planner will explore all of them
   to try to find the most efficient query plan.
  </para>

  <para>
   When a query only involves two or three tables, there aren't many join
   orders to worry about.  But the number of possible join orders grows
   exponentially as the number of tables expands.  Beyond ten or so input
   tables it's no longer practical to do an exhaustive search of all the
   possibilities, and even for six or seven tables planning may take an
   annoyingly long time.  When there are too many input tables, the
   <productname>Postgres</productname> planner will switch from exhaustive
   search to a <firstterm>genetic</firstterm> probabilistic search
   through a limited number of possibilities.  (The switchover threshold is
   set by the GEQO_THRESHOLD run-time
   parameter described in the <citetitle>Administrator's Guide</citetitle>.)
   The genetic search takes less time, but it won't
   necessarily find the best possible plan.
  </para>

  <para>
   When the query involves outer joins, the planner has much less freedom
   than it does for plain (inner) joins. For example, consider
    <programlisting>
SELECT * FROM a LEFT JOIN (b JOIN c ON (b.ref = c.id)) ON (a.id = b.id);
    </programlisting>
   Although this query's restrictions are superficially similar to the
   previous example, the semantics are different because a row must be
   emitted for each row of A that has no matching row in the join of B and C.
   Therefore the planner has no choice of join order here: it must join
   B to C and then join A to that result.  Accordingly, this query takes
   less time to plan than the previous query.
  </para>

  <para>
   In <productname>Postgres</productname> 7.1, the planner treats all
   explicit JOIN syntaxes as constraining the join order, even though
   it is not logically necessary to make such a constraint for inner
   joins.  Therefore, although all of these queries give the same result:
    <programlisting>
SELECT * FROM a,b,c WHERE a.id = b.id AND b.ref = c.id;
SELECT * FROM a CROSS JOIN b CROSS JOIN c WHERE a.id = b.id AND b.ref = c.id;
SELECT * FROM a JOIN (b JOIN c ON (b.ref = c.id)) ON (a.id = b.id);
    </programlisting>
   the second and third take less time to plan than the first.  This effect
   is not worth worrying about for only three tables, but it can be a
   lifesaver with many tables.
  </para>

  <para>
   You do not need to constrain the join order completely in order to
   cut search time, because it's OK to use JOIN operators in a plain
   FROM list.  For example,
    <programlisting>
SELECT * FROM a CROSS JOIN b, c, d, e WHERE ...;
    </programlisting>
   forces the planner to join A to B before joining them to other tables,
   but doesn't constrain its choices otherwise.  In this example, the
   number of possible join orders is reduced by a factor of 5.
  </para>

  <para>
   If you have a mix of outer and inner joins in a complex query, you
   might not want to constrain the planner's search for a good ordering
   of inner joins inside an outer join.  You can't do that directly in the
   JOIN syntax, but you can get around the syntactic limitation by using
   subselects.  For example,
    <programlisting>
SELECT * FROM d LEFT JOIN
        (SELECT * FROM a, b, c WHERE ...) AS ss
        ON (...);
    </programlisting>
   Here, joining D must be the last step in the query plan, but the
   planner is free to consider various join orders for A,B,C.
  </para>

  <para>
   Constraining the planner's search in this way is a useful technique
   both for reducing planning time and for directing the planner to a
   good query plan.  If the planner chooses a bad join order by default,
   you can force it to choose a better order via JOIN syntax --- assuming
   that you know of a better order, that is.  Experimentation is recommended.
  </para>
 </sect1>

 <sect1 id="populate">
  <title>Populating a Database</title>

  <para>
   One may need to do a large number of table insertions when first
   populating a database. Here are some tips and techniques for making that as
   efficient as possible.
  </para>

  <sect2 id="disable-autocommit">
   <title>Disable Auto-commit</title>

   <para>
    Turn off auto-commit and just do one commit at
    the end.  Otherwise <productname>Postgres</productname> is doing a
    lot of work for each record
    added.  In general when you are doing bulk inserts, you want
    to turn off some of the database features to gain speed.
   </para>
  </sect2>

  <sect2 id="populate-copy-from">
   <title>Use COPY FROM</title>

   <para>
    Use <command>COPY FROM STDIN</command> to load all the records in one
    command, instead
    of a series of INSERT commands.  This reduces parsing, planning, etc
    overhead a great deal. If you do this then it's not necessary to fool
    around with autocommit, since it's only one command anyway.
   </para>
  </sect2>

  <sect2 id="populate-rm-indices">
   <title>Remove Indices</title>

   <para>
    If you are loading a freshly created table, the fastest way is to
    create the table, bulk-load with COPY, then create any indexes needed
    for the table.  Creating an index on pre-existing data is quicker than
    updating it incrementally as each record is loaded.
   </para>

   <para>
    If you are augmenting an existing table, you can <command>DROP
     INDEX</command>, load the table, then recreate the index. Of
    course, the database performance for other users may be adversely 
    affected during the time that the index is missing.
   </para>
  </sect2>
  </sect1>

 </chapter>

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