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Allow \watch queries to stop on minimum rows returned

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When running a repeat query with \watch in psql, it can be
helpful to be able to stop the watch process when the query
no longer returns the expected amount of rows.  An example
would be to watch for the presence of a certain event in
pg_stat_activity and stopping when the event is no longer
present, or to watch an index creation and stop when the
index is created.

This adds a min_rows=MIN parameter to \watch which can be
set to a non-negative integer, and the watch query will
stop executing when it returns less than MIN rows.

Author: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAKAnmmKStATuddYxP71L+p0DHtp9Rvjze3XRoy0Dyw67VQ45UA@mail.gmail.com
This commit is contained in:
Daniel Gustafsson
2023-08-29 11:30:11 +02:00
parent 95fff2abee
commit f347ec76e2
6 changed files with 72 additions and 15 deletions
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7
doc/src/sgml/ref/psql-ref.sgml
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@ -3566,13 +3566,14 @@ testdb=&gt; <userinput>\setenv LESS -imx4F</userinput>
<varlistentry id="app-psql-meta-command-watch">
<term><literal>\watch [ i[nterval]=<replaceable class="parameter">seconds</replaceable> ] [ c[ount]=<replaceable class="parameter">times</replaceable> ] [ <replaceable class="parameter">seconds</replaceable> ]</literal></term>
<term><literal>\watch [ i[nterval]=<replaceable class="parameter">seconds</replaceable> ] [ c[ount]=<replaceable class="parameter">times</replaceable> ] [ m[in_rows]=<replaceable class="parameter">rows</replaceable> ] [ <replaceable class="parameter">seconds</replaceable> ]</literal></term>
<listitem>
<para>
Repeatedly execute the current query buffer (as <literal>\g</literal> does)
until interrupted, or the query fails, or the execution count limit
(if given) is reached. Wait the specified number of
seconds (default 2) between executions. For backwards compatibility,
(if given) is reached, or the query no longer returns the minimum number
of rows. Wait the specified number of seconds (default 2) between executions.
For backwards compatibility,
<replaceable class="parameter">seconds</replaceable> can be specified
with or without an <literal>interval=</literal> prefix.
Each query result is