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>Chapter 11. Indexes</TD
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><H1
CLASS="SECT1"
><A
NAME="INDEXES-PARTIAL"
>11.8. Partial Indexes</A
></H1
><P
> A <I
CLASS="FIRSTTERM"
>partial index</I
> is an index built over a
subset of a table; the subset is defined by a conditional
expression (called the <I
CLASS="FIRSTTERM"
>predicate</I
> of the
partial index). The index contains entries only for those table
rows that satisfy the predicate. Partial indexes are a specialized
feature, but there are several situations in which they are useful.
</P
><P
> One major reason for using a partial index is to avoid indexing common
values. Since a query searching for a common value (one that
accounts for more than a few percent of all the table rows) will not
use the index anyway, there is no point in keeping those rows in the
index at all. This reduces the size of the index, which will speed
up those queries that do use the index. It will also speed up many table
update operations because the index does not need to be
updated in all cases. <A
HREF="indexes-partial.html#INDEXES-PARTIAL-EX1"
>Example 11-1</A
> shows a
possible application of this idea.
</P
><DIV
CLASS="EXAMPLE"
><A
NAME="INDEXES-PARTIAL-EX1"
></A
><P
><B
>Example 11-1. Setting up a Partial Index to Exclude Common Values</B
></P
><P
> Suppose you are storing web server access logs in a database.
Most accesses originate from the IP address range of your organization but
some are from elsewhere (say, employees on dial-up connections).
If your searches by IP are primarily for outside accesses,
you probably do not need to index the IP range that corresponds to your
organization's subnet.
</P
><P
> Assume a table like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE access_log (
url varchar,
client_ip inet,
...
);</PRE
><P>
</P
><P
> To create a partial index that suits our example, use a command
such as this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX access_log_client_ip_ix ON access_log (client_ip)
WHERE NOT (client_ip > inet '192.168.100.0' AND
client_ip < inet '192.168.100.255');</PRE
><P>
</P
><P
> A typical query that can use this index would be:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM access_log
WHERE url = '/index.html' AND client_ip = inet '212.78.10.32';</PRE
><P>
A query that cannot use this index is:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM access_log
WHERE client_ip = inet '192.168.100.23';</PRE
><P>
</P
><P
> Observe that this kind of partial index requires that the common
values be predetermined, so such partial indexes are best used for
data distributions that do not change. The indexes can be recreated
occasionally to adjust for new data distributions, but this adds
maintenance effort.
</P
></DIV
><P
> Another possible use for a partial index is to exclude values from the
index that the
typical query workload is not interested in; this is shown in <A
HREF="indexes-partial.html#INDEXES-PARTIAL-EX2"
>Example 11-2</A
>. This results in the same
advantages as listed above, but it prevents the
<SPAN
CLASS="QUOTE"
>"uninteresting"</SPAN
> values from being accessed via that
index, even if an index scan might be profitable in that
case. Obviously, setting up partial indexes for this kind of
scenario will require a lot of care and experimentation.
</P
><DIV
CLASS="EXAMPLE"
><A
NAME="INDEXES-PARTIAL-EX2"
></A
><P
><B
>Example 11-2. Setting up a Partial Index to Exclude Uninteresting Values</B
></P
><P
> If you have a table that contains both billed and unbilled orders,
where the unbilled orders take up a small fraction of the total
table and yet those are the most-accessed rows, you can improve
performance by creating an index on just the unbilled rows. The
command to create the index would look like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX orders_unbilled_index ON orders (order_nr)
WHERE billed is not true;</PRE
><P>
</P
><P
> A possible query to use this index would be:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM orders WHERE billed is not true AND order_nr < 10000;</PRE
><P>
However, the index can also be used in queries that do not involve
<TT
CLASS="STRUCTFIELD"
>order_nr</TT
> at all, e.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM orders WHERE billed is not true AND amount > 5000.00;</PRE
><P>
This is not as efficient as a partial index on the
<TT
CLASS="STRUCTFIELD"
>amount</TT
> column would be, since the system has to
scan the entire index. Yet, if there are relatively few unbilled
orders, using this partial index just to find the unbilled orders
could be a win.
</P
><P
> Note that this query cannot use this index:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM orders WHERE order_nr = 3501;</PRE
><P>
The order 3501 might be among the billed or unbilled
orders.
</P
></DIV
><P
> <A
HREF="indexes-partial.html#INDEXES-PARTIAL-EX2"
>Example 11-2</A
> also illustrates that the
indexed column and the column used in the predicate do not need to
match. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports partial
indexes with arbitrary predicates, so long as only columns of the
table being indexed are involved. However, keep in mind that the
predicate must match the conditions used in the queries that
are supposed to benefit from the index. To be precise, a partial
index can be used in a query only if the system can recognize that
the <TT
CLASS="LITERAL"
>WHERE</TT
> condition of the query mathematically implies
the predicate of the index.
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does not have a sophisticated
theorem prover that can recognize mathematically equivalent
expressions that are written in different forms. (Not
only is such a general theorem prover extremely difficult to
create, it would probably be too slow to be of any real use.)
The system can recognize simple inequality implications, for example
<SPAN
CLASS="QUOTE"
>"x < 1"</SPAN
> implies <SPAN
CLASS="QUOTE"
>"x < 2"</SPAN
>; otherwise
the predicate condition must exactly match part of the query's
<TT
CLASS="LITERAL"
>WHERE</TT
> condition
or the index will not be recognized as usable. Matching takes
place at query planning time, not at run time. As a result,
parameterized query clauses do not work with a partial index. For
example a prepared query with a parameter might specify
<SPAN
CLASS="QUOTE"
>"x < ?"</SPAN
> which will never imply
<SPAN
CLASS="QUOTE"
>"x < 2"</SPAN
> for all possible values of the parameter.
</P
><P
> A third possible use for partial indexes does not require the
index to be used in queries at all. The idea here is to create
a unique index over a subset of a table, as in <A
HREF="indexes-partial.html#INDEXES-PARTIAL-EX3"
>Example 11-3</A
>. This enforces uniqueness
among the rows that satisfy the index predicate, without constraining
those that do not.
</P
><DIV
CLASS="EXAMPLE"
><A
NAME="INDEXES-PARTIAL-EX3"
></A
><P
><B
>Example 11-3. Setting up a Partial Unique Index</B
></P
><P
> Suppose that we have a table describing test outcomes. We wish
to ensure that there is only one <SPAN
CLASS="QUOTE"
>"successful"</SPAN
> entry for
a given subject and target combination, but there might be any number of
<SPAN
CLASS="QUOTE"
>"unsuccessful"</SPAN
> entries. Here is one way to do it:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE tests (
subject text,
target text,
success boolean,
...
);
CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target)
WHERE success;</PRE
><P>
This is a particularly efficient approach when there are few
successful tests and many unsuccessful ones.
</P
></DIV
><P
> Finally, a partial index can also be used to override the system's
query plan choices. Also, data sets with peculiar
distributions might cause the system to use an index when it really
should not. In that case the index can be set up so that it is not
available for the offending query. Normally,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> makes reasonable choices about index
usage (e.g., it avoids them when retrieving common values, so the
earlier example really only saves index size, it is not required to
avoid index usage), and grossly incorrect plan choices are cause
for a bug report.
</P
><P
> Keep in mind that setting up a partial index indicates that you
know at least as much as the query planner knows, in particular you
know when an index might be profitable. Forming this knowledge
requires experience and understanding of how indexes in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> work. In most cases, the advantage of a
partial index over a regular index will be minimal.
</P
><P
> More information about partial indexes can be found in <A
HREF="biblio.html#STON89B"
><I
><A
HREF="http://db.cs.berkeley.edu/papers/ERL-M89-17.pdf"
TARGET="_top"
> The case for partial indexes
</A
></I
></A
>, <A
HREF="biblio.html#OLSON93"
><I
>Partial indexing in POSTGRES: research project</I
></A
>, and <A
HREF="biblio.html#SESHADRI95"
><I
>Generalized Partial Indexes
<A
HREF="http://citeseer.ist.psu.edu/seshadri95generalized.html"
TARGET="_top"
>(cached version)
</A
></I
></A
>.
</P
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