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Redesign DISTINCT ON as discussed in pgsql-sql 1/25/00: syntax is now

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SELECT DISTINCT ON (expr [, expr ...]) targetlist ...
and there is a check to make sure that the user didn't specify an ORDER BY
that's incompatible with the DISTINCT operation.
Reimplement nodeUnique and nodeGroup to use the proper datatype-specific
equality function for each column being compared --- they used to do
bitwise comparisons or convert the data to text strings and strcmp().
(To add insult to injury, they'd look up the conversion functions once
for each tuple...)  Parse/plan representation of DISTINCT is now a list
of SortClause nodes.
initdb forced by querytree change...
This commit is contained in:
Tom Lane
2000-01-27 18:11:50 +00:00
parent 3f0074e403
commit dd979f66be
32 changed files with 638 additions and 576 deletions
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29
doc/src/sgml/ref/select.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.23 1999/12/13 17:39:38 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.24 2000/01/27 18:11:25 tgl Exp $
Postgres documentation
-->
@ -22,7 +22,7 @@ Postgres documentation
<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
SELECT [ ALL | DISTINCT [ ON <replaceable class="PARAMETER">column</replaceable> ] ]
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
<replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
[ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
[ FROM <replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
@ -201,16 +201,29 @@ SELECT [ ALL | DISTINCT [ ON <replaceable class="PARAMETER">column</replaceable>
</para>
<para>
<command>DISTINCT</command> will eliminate all duplicate rows from the
<command>DISTINCT</command> will eliminate duplicate rows from the
result.
<command>DISTINCT ON <replaceable class="PARAMETER">column</replaceable></command>
will eliminate all duplicates in the specified column; this is
similar to using
<command>GROUP BY <replaceable class="PARAMETER">column</replaceable></command>.
<command>ALL</command> will return all candidate rows,
<command>ALL</command> (the default) will return all candidate rows,
including duplicates.
</para>
<para>
<command>DISTINCT ON</command> eliminates rows that match on all the
specified expressions, keeping only the first row of each set of
duplicates. Note that "the first row" of each set is unpredictable
unless <command>ORDER BY</command> is used to ensure that the desired
row appears first. For example,
<programlisting>
SELECT DISTINCT ON (location) location, time, report
FROM weatherReports
ORDER BY location, time DESC;
</programlisting>
retrieves the most recent weather report for each location. But if
we had not used ORDER BY to force descending order of time values
for each location, we'd have gotten a report of unpredictable age
for each location.
</para>
<para>
The GROUP BY clause allows a user to divide a table
conceptually into groups.