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Add more info on regex's using INDEX.

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Bruce Momjian 2002-06-25 03:32:31 +00:00
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2 changed files with 44 additions and 13 deletions
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20
doc/FAQ
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@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Jun 24 21:45:50 EDT 2002
Last updated: Mon Jun 24 23:32:16 EDT 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -747,10 +747,20 @@
LIMIT 1
When using wild-card operators such as LIKE or ~, indexes can only be
used if the default C local is used during initdb and the beginning of
the search is anchored to the start of the string. Therefore, to use
indexes, LIKE patterns must not start with %, and ~(regular
expression) patterns must start with ^.
used in certain circumstances:
* The beginning of the search string must be anchored to the start
of the string, i.e.:
* LIKE patterns must not start with %.
* ~(regular expression) patterns must start with ^.
The search string can not start with a character class, e.g. [a-e].
Case-insensitive searches like ILIKE and ~* can not be used.
Instead, use functional indexes, which are described later in this
FAQ.
The default C local must have been used during initdb.
4.9) How do I see how the query optimizer is evaluating my query?

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doc/src/FAQ/FAQ.html
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@ -14,7 +14,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Mon Jun 24 21:45:50 EDT 2002</P>
<P>Last updated: Mon Jun 24 23:32:16 EDT 2002</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@ -282,6 +282,7 @@
subscribe
end
</PRE>
Digests are sent out to members of this list whenever the main list
has received around 30k of messages.
@ -293,6 +294,7 @@
subscribe
end
</PRE>
There is also a developers discussion mailing list available. To
subscribe to this list, send email to <A href=
"mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
@ -860,6 +862,7 @@
Maximum number of columns in a table? 250-1600 depending on column types
Maximum number of indexes on a table? unlimited
</PRE>
Of course, these are not actually unlimited, but limited to
available disk space and memory/swap space. Performance may suffer
when these values get unusually large.
@ -951,11 +954,24 @@
</PRE>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
<I>~</I>, indexes can only be used if the default C local is used
during initdb and the beginning of the search
is anchored to the start of the string. Therefore, to use indexes,
<SMALL>LIKE</SMALL> patterns must not start with <I>%</I>, and
<I>~</I>(regular expression) patterns must start with <I>^</I>.</P>
<I>~</I>, indexes can only be used in certain circumstances:
<UL>
<LI>The beginning of the search string must be anchored to the start
of the string, i.e.:</LI>
<UL>
<LI><SMALL>LIKE</SMALL> patterns must not start with <I>%.</I></LI>
<LI><I>~</I>(regular expression) patterns must start with
<I>^.</I></LI>
</UL>
<LI>The search string can not start with a character class,
e.g. [a-e].</LI>
<LI>Case-insensitive searches like <SMALL>ILIKE</SMALL> and
<I>~*</I> can not be used. Instead, use functional
indexes, which are described later in this FAQ.</LI>
<LI>The default <I>C</I> local must have been used during
<i>initdb.</i></LI>
</UL>
<P>
<H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
evaluating my query?</H4>
@ -1010,13 +1026,12 @@ during initdb and the beginning of the search
SELECT *
FROM tab
WHERE lower(col) = 'abc'
</PRE>
This will not use an standard index. However, if you create a
functional index, it will be used:
<PRE>
CREATE INDEX tabindex on tab (lower(col));
</PRE>
<H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
@ -1066,6 +1081,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
name TEXT
);
</PRE>
is automatically translated into this:
<PRE>
CREATE SEQUENCE person_id_seq;
@ -1075,6 +1091,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
);
CREATE UNIQUE INDEX person_id_key ON person ( id );
</PRE>
See the <I>create_sequence</I> manual page for more information
about sequences. You can also use each row's <I>OID</I> field as a
unique value. However, if you need to dump and reload the database,
@ -1093,6 +1110,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
new_id = output of "SELECT nextval('person_id_seq')"
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
</PRE>
You would then also have the new value stored in
<CODE>new_id</CODE> for use in other queries (e.g., as a foreign
key to the <CODE>person</CODE> table). Note that the name of the
@ -1108,6 +1126,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
INSERT INTO person (name) VALUES ('Blaise Pascal');
new_id = output of "SELECT currval('person_id_seq')";
</PRE>
Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
returned from the <SMALL>INSERT</SMALL> statement to look up the
default value, though this is probably the least portable approach.
@ -1215,6 +1234,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
ulimit -d 262144
limit datasize 256m
</PRE>
Depending on your shell, only one of these may succeed, but it will
set your process data segment limit much higher and perhaps allow
the query to complete. This command applies to the current process,
@ -1273,6 +1293,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
</CODE>
</PRE>
We hope to fix this limitation in a future release.
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>