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Doc: clarify behavior of row-limit arguments in the PLs' SPI wrappers.

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plperl, plpython, and pltcl all provide query-execution functions
that are thin wrappers around SPI_execute() or its variants.
The SPI functions document their row-count limit arguments clearly,
as "maximum number of rows to return, or 0 for no limit".  However
the PLs' documentation failed to explain this special behavior of
zero, so that a reader might well assume it means "fetch zero
rows".  Improve that.

Daniel Gustafsson and Tom Lane, per report from Kieran McCusker

Discussion: https://postgr.es/m/CAGgUQ6H6qYScctOhktQ9HLFDDoafBKHyUgJbZ6q_dOApnzNTXg@mail.gmail.com
This commit is contained in:
Tom Lane 2023-05-02 17:55:01 -04:00
parent 77ea05406c
commit 23c7aa865b
3 changed files with 32 additions and 10 deletions
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21
doc/src/sgml/plperl.sgml
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@ -441,7 +441,7 @@ use strict;
<variablelist>
<varlistentry>
<term>
<literal><function>spi_exec_query</function>(<replaceable>query</replaceable> [, <replaceable>max-rows</replaceable>])</literal>
<literal><function>spi_exec_query</function>(<replaceable>query</replaceable> [, <replaceable>limit</replaceable>])</literal>
<indexterm>
<primary>spi_exec_query</primary>
<secondary>in PL/Perl</secondary>
@ -449,9 +449,17 @@ use strict;
</term>
<listitem>
<para>
<literal>spi_exec_query</literal> executes an SQL command and
returns the entire row set as a reference to an array of hash
references. <emphasis>You should only use this command when you know
<function>spi_exec_query</function> executes an SQL command and
returns the entire row set as a reference to an array of hash references.
If <replaceable>limit</replaceable> is specified and is greater than zero,
then <function>spi_exec_query</function> retrieves at
most <replaceable>limit</replaceable> rows, much as if the query included
a <literal>LIMIT</literal> clause. Omitting <replaceable>limit</replaceable>
or specifying it as zero results in no row limit.
</para>
<para>
<emphasis>You should only use this command when you know
that the result set will be relatively small.</emphasis> Here is an
example of a query (<command>SELECT</command> command) with the
optional maximum number of rows:
@ -643,7 +651,10 @@ $plan = spi_prepare('SELECT * FROM test WHERE id &gt; $1 AND name = $2',
by <literal>spi_exec_query</literal>, or in <literal>spi_query_prepared</literal> which returns a cursor
exactly as <literal>spi_query</literal> does, which can be later passed to <literal>spi_fetchrow</literal>.
The optional second parameter to <literal>spi_exec_prepared</literal> is a hash reference of attributes;
the only attribute currently supported is <literal>limit</literal>, which sets the maximum number of rows returned by a query.
the only attribute currently supported is <literal>limit</literal>, which
sets the maximum number of rows returned from the query.
Omitting <literal>limit</literal> or specifying it as zero results in no
row limit.
</para>
<para>

13
doc/src/sgml/plpython.sgml
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@ -789,7 +789,7 @@ $$ LANGUAGE plpython3u;
<variablelist>
<varlistentry>
<term><literal>plpy.<function>execute</function>(<replaceable>query</replaceable> [, <replaceable>max-rows</replaceable>])</literal></term>
<term><literal>plpy.<function>execute</function>(<replaceable>query</replaceable> [, <replaceable>limit</replaceable>])</literal></term>
<listitem>
<para>
Calling <function>plpy.execute</function> with a query string and an
@ -797,6 +797,15 @@ $$ LANGUAGE plpython3u;
be returned in a result object.
</para>
<para>
If <replaceable>limit</replaceable> is specified and is greater than
zero, then <function>plpy.execute</function> retrieves at
most <replaceable>limit</replaceable> rows, much as if the query
included a <literal>LIMIT</literal>
clause. Omitting <replaceable>limit</replaceable> or specifying it as
zero results in no row limit.
</para>
<para>
The result object emulates a list or dictionary object. The result
object can be accessed by row number and column name. For example:
@ -887,7 +896,7 @@ foo = rv[i]["my_column"]
<varlistentry>
<term><literal>plpy.<function>prepare</function>(<replaceable>query</replaceable> [, <replaceable>argtypes</replaceable>])</literal></term>
<term><literal>plpy.<function>execute</function>(<replaceable>plan</replaceable> [, <replaceable>arguments</replaceable> [, <replaceable>max-rows</replaceable>]])</literal></term>
<term><literal>plpy.<function>execute</function>(<replaceable>plan</replaceable> [, <replaceable>arguments</replaceable> [, <replaceable>limit</replaceable>]])</literal></term>
<listitem>
<para>
<indexterm><primary>preparing a query</primary><secondary>in PL/Python</secondary></indexterm>

8
doc/src/sgml/pltcl.sgml
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@ -341,9 +341,11 @@ $$ LANGUAGE pltcl;
</para>
<para>
The optional <literal>-count</literal> value tells
<function>spi_exec</function> the maximum number of rows
to process in the command. The effect of this is comparable to
setting up a query as a cursor and then saying <literal>FETCH <replaceable>n</replaceable></literal>.
<function>spi_exec</function> to stop
once <replaceable>n</replaceable> rows have been retrieved,
much as if the query included a <literal>LIMIT</literal> clause.
If <replaceable>n</replaceable> is zero, the query is run to
completion, the same as when <literal>-count</literal> is omitted.
</para>
<para>
If the command is a <command>SELECT</command> statement, the values of the