<Chapter Id="pg-options">

<Chapter Id="pg-options-dev">

<DocInfo>

<AuthorGroup>

<Author>

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$Header: /cvsroot/pgsql/doc/src/sgml/postgres.sgml,v 1.22 1999/05/20 05:39:27 thomas Exp $

$Header: /cvsroot/pgsql/doc/src/sgml/postgres.sgml,v 1.23 1999/05/22 02:27:24 thomas Exp $

Postgres integrated documentation.

Other subset docs should be copied and shrunk from here.

thomas 1998-02-23

$Log: postgres.sgml,v $

Revision 1.23 1999/05/22 02:27:24 thomas

Finish initial markup of cvs.sgml, and include it in the programmer's guide

and the integrated doc. Clean up other markup.

Revision 1.22 1999/05/20 05:39:27 thomas

Rearrange and consolidate the Admin Guide.

Add reference pages for utilities and remove standalone chapters for same.

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Additional related information.

</Para>

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&docguide;

&datetime;

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&biblio;

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Postgres Programmer's Guide.

- thomas 1998-10-27

$Log: programmer.sgml,v $

Revision 1.15 1999/05/22 02:27:24 thomas

Finish initial markup of cvs.sgml, and include it in the programmer's guide

and the integrated doc. Clean up other markup.

Revision 1.14 1999/05/20 05:39:27 thomas

Rearrange and consolidate the Admin Guide.

Add reference pages for utilities and remove standalone chapters for same.

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</para>

</sect1>

</Para>

</sect1>

<Sect1>

<Title>Querying a Class</Title>

<parameter>D<subscript>i</subscript></parameter>,

for each attribute

<parameter>A<subscript>i</subscript></parameter>,

1 &lt;&equal; <literal>i</literal> &lt;&equal; <literal>k</literal>,

1 &lt;&equals; <literal>i</literal> &lt;&equals; <literal>k</literal>,

where the values of the attributes are taken from. We often write

a relation scheme as

<literal>R(<parameter>A<subscript>1</subscript></parameter>,

integers. We define this by assigning a data type to each

attribute. The type of <classname>SNAME</classname> will be

<type>VARCHAR(20)</type> (this is the <acronym>SQL</acronym> type

for character strings of length &lt;&equal; 20),

for character strings of length &lt;&equals; 20),

the type of <classname>SNO</classname> will be

<type>INTEGER</type>. With the assignment of a data type we also have selected

a domain for an attribute. The domain of <classname>SNAME</classname> is the set of all

character strings of length &lt;&equal; 20,

character strings of length &lt;&equals; 20,

the domain of <classname>SNO</classname> is the set of

all integer numbers.

</para>

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$Header: /cvsroot/pgsql/doc/src/sgml/Attic/user.sgml,v 1.9 1999/05/20 05:39:29 thomas Exp $

$Header: /cvsroot/pgsql/doc/src/sgml/Attic/user.sgml,v 1.10 1999/05/22 02:27:25 thomas Exp $

Postgres User's Manual.

Derived from postgres.sgml.

thomas 1998-02-24

$Log: user.sgml,v $

Revision 1.10 1999/05/22 02:27:25 thomas

Finish initial markup of cvs.sgml, and include it in the programmer's guide

and the integrated doc. Clean up other markup.

Revision 1.9 1999/05/20 05:39:29 thomas

Rearrange and consolidate the Admin Guide.

Add reference pages for utilities and remove standalone chapters for same.

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&manage;

&storage;

&commands;

&datetime;

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&contacts;

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<para>

Providing NEGATOR is very helpful to the query optimizer since

it allows expressions like NOT (x = y) to be simplified into

x<> y. This comes up more often than you might think, because

x&lt;&gt; y. This comes up more often than you might think, because

NOTs can be inserted as a consequence of other rearrangements.

</para>

These are the standard restriction estimators:

<ProgramListing>

eqselfor =

neqselfor<>

intltselfor< or<=

intgtselfor> or>=

neqselfor&lt;&gt;

intltselfor&lt; or&lt;=

intgtselfor&gt; or&gt;=

</ProgramListing>

It might seem a little odd that these are the categories, but they

make sense if you think about it. '=' will typically accept only

a small fraction of the rows in a table; '<>' will typically reject

only a small fraction. '<' will accept a fraction that depends on

a small fraction of the rows in a table; '&lt;&gt;' will typically reject

only a small fraction. '&lt;' will accept a fraction that depends on

where the given constant falls in the range of values for that table

column (which, it just so happens, is information collected by

VACUUM ANALYZE and made available to the selectivity estimator).

'<=' will accept a slightly larger fraction than '<' for the same

'&lt;=' will accept a slightly larger fraction than '&lt;' for the same

comparison constant, but they're close enough to not be worth

distinguishing, especially since we're not likely to do better than a

rough guess anyhow. Similar remarks apply to '>' and '>='.

rough guess anyhow. Similar remarks apply to '&gt;' and '&gt;='.

</para>

<para>

matching operators (~, ~*, etc) use eqsel on the assumption that they'll

usually only match a small fraction of the entries in a table.

</para>

</sect2>

<sect2>

<title>JOIN</title>

<sect2>

<title>JOIN</title>

<para>

The JOIN clause, if provided, names a join selectivity

estimation function for the operator (note that this is a function

name, not an operator name). JOIN clauses only make sense for

binary operators that return boolean. The idea behind a join

selectivity estimator is to guess what fraction of the rows in a

pair of tables will satisfy a WHERE-clause condition of the form

<ProgramListing>

<para>

The JOIN clause, if provided, names a join selectivity

estimation function for the operator (note that this is a function

name, not an operator name). JOIN clauses only make sense for

binary operators that return boolean. The idea behind a join

selectivity estimator is to guess what fraction of the rows in a

pair of tables will satisfy a WHERE-clause condition of the form

<ProgramListing>

table1.field1 OP table2.field2

</ProgramListing>

for the current operator. As with the RESTRICT clause, this helps

the optimizer very substantially by letting it figure out which

of several possible join sequences is likely to take the least work.

</para>

</ProgramListing>

for the current operator. As with the RESTRICT clause, this helps

the optimizer very substantially by letting it figure out which

of several possible join sequences is likely to take the least work.

</para>

<para>

As before, this chapter will make no attempt to explain how to write

a join selectivity estimator function, but will just suggest that

you use one of the standard estimators if one is applicable:

<ProgramListing>

<para>

As before, this chapter will make no attempt to explain how to write

a join selectivity estimator function, but will just suggest that

you use one of the standard estimators if one is applicable:

<ProgramListing>

eqjoinselfor =

neqjoinselfor <>

intltjoinselfor < or <=

intgtjoinselfor > or >=

</ProgramListing>

</para>

neqjoinselfor &lt;&gt;

intltjoinselfor &lt; or &lt;=

intgtjoinselfor &gt; or &gt;=

</ProgramListing>

</para>

</sect2>

</sect2>

<sect2>

<title>HASHES</title>

<sect2>

<title>HASHES</title>

<para>

The HASHES clause, if present, tells the system that it is OK to

use the hash join method for a join based on this operator. HASHES

only makes sense for binary operators that return boolean --- and

in practice, the operator had better be equality for some data type.

</para>

<para>

The HASHES clause, if present, tells the system that it is OK to

use the hash join method for a join based on this operator. HASHES

only makes sense for binary operators that return boolean --- and

in practice, the operator had better be equality for some data type.

</para>

<para>

The assumption underlying hash join is that the join operator can