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docs: adjust multi-column most-common-value statistics

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This commit adds a mention that the order of columns specified during
multi-column most-common-value statistics is insignificant, and tries to
simplify examples.

Discussion: https://postgr.es/m/20190828162238.GA8360@momjian.us

Backpatch-through: 12
This commit is contained in:
Bruce Momjian 2019-09-30 13:44:22 -04:00
parent 9d42df74a3
commit 737f226842
2 changed files with 5 additions and 4 deletions
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6
doc/src/sgml/perform.sgml
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@ -1172,7 +1172,7 @@ WHERE tablename = 'road';
<para>
Here is an example of collecting functional-dependency statistics:
<programlisting>
CREATE STATISTICS stts (dependencies) ON zip, city FROM zipcodes;
CREATE STATISTICS stts (dependencies) ON city, zip FROM zipcodes;
ANALYZE zipcodes;
@ -1262,7 +1262,7 @@ SELECT * FROM zipcodes WHERE city = 'San Francisco' AND zip = '90210';
Continuing the previous example, the n-distinct counts in a
table of ZIP codes might look like the following:
<programlisting>
CREATE STATISTICS stts2 (ndistinct) ON zip, state, city FROM zipcodes;
CREATE STATISTICS stts2 (ndistinct) ON city, state, zip FROM zipcodes;
ANALYZE zipcodes;
@ -1317,7 +1317,7 @@ nd | {"1, 2": 33178, "1, 5": 33178, "2, 5": 27435, "1, 2, 5": 33178}
a function is required for inspection of MCV contents):
<programlisting>
CREATE STATISTICS stts3 (mcv) ON state, city FROM zipcodes;
CREATE STATISTICS stts3 (mcv) ON city, state FROM zipcodes;
ANALYZE zipcodes;

3
doc/src/sgml/ref/create_statistics.sgml
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@ -98,7 +98,8 @@ CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_na
<listitem>
<para>
The name of a table column to be covered by the computed statistics.
At least two column names must be given.
At least two column names must be given; the order of the column names
is insignificant.
</para>
</listitem>
</varlistentry>