Tag: scripting

Waiting for PostgreSQL 17 – Allow \watch queries to stop on minimum rows returned

On 29th of August 2023, Daniel Gustafsson committed patch:

Allow \watch queries to stop on minimum rows returned
 
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

Continue reading Waiting for PostgreSQL 17 – Allow \watch queries to stop on minimum rows returned

How to run short ALTER TABLE without long locking concurrent queries

Recently I've seen case like:

  1. application had to add column to table.
  2. application ran ALTER TABLE ADD COLUMN (without default!)
  3. everything stopped for many MINUTES

Why? How to avoid the problem?

Continue reading How to run short ALTER TABLE without long locking concurrent queries

Waiting for PostgreSQL 11 – Add psql variables to track success/failure of SQL queries.

On 12nd of September 2017, Tom Lane committed patch:

Add psql variables to track success/failure of SQL queries.
 
 
This patch adds ERROR, SQLSTATE, and ROW_COUNT, which are updated after
every query, as well as LAST_ERROR_MESSAGE and LAST_ERROR_SQLSTATE,
which are updated only when a query fails.  The expected usage of these
is for scripting.
 
Fabien Coelho, reviewed by Pavel Stehule
 
Discussion: https://postgr.es/m/alpine.DEB.2.20..12290@lancre

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Waiting for PostgreSQL 10 – Support \if … \elif … \else … \endif in psql scripting.

On 30th of March 2017, Tom Lane committed patch:

Support \if ... \elif ... \else ... \endif in psql scripting.
 
This patch adds nestable conditional blocks to psql.  The control
structure feature per se is complete, but the boolean expressions
understood by \if and \elif are pretty primitive; basically, after
variable substitution and backtick expansion, the result has to be
"true" or "false" or one of the other standard spellings of a boolean
value.  But that's enough for many purposes, since you can always
do the heavy lifting on the server side; and we can extend it later.
 
Along the way, pay down some of the technical debt that had built up
around psql/command.c:
* Refactor exec_command() into a function per command, instead of
being a 1500-line monstrosity.  This makes the file noticeably longer
because of repetitive function header/trailer overhead, but it seems
much more readable.
* Teach psql_get_variable() and psqlscanslash.l to suppress variable
substitution and backtick expansion on the basis of the conditional
stack state, thereby allowing removal of the OT_NO_EVAL kluge.
* Fix the no-doubt-once-expedient hack of sometimes silently substituting
mainloop.c's previous_buf for query_buf when calling HandleSlashCmds.
(It's a bit remarkable that commands like \r worked at all with that.)
Recall of a previous query is now done explicitly in the slash commands
where that should happen.
 
Corey Huinker, reviewed by Fabien Coelho, further hacking by me
 
Discussion: https://postgr.es/m/CADkLM=c94OSRTnat=LX0ivNq4pxDNeoomFfYvBKM5N_xfmLtAA@mail.gmail.com

Continue reading Waiting for PostgreSQL 10 – Support \if … \elif … \else … \endif in psql scripting.