Waiting for PostgreSQL 12 – Generated columns

On 30th of March 2019, Peter Eisentraut committed patch:

Generated columns
 
This is an SQL-standard feature that allows creating columns that are
computed from expressions rather than assigned, similar to a view or
materialized view but on a column basis.
 
This implements one kind of generated column: stored (computed on
write).  Another kind, virtual (computed on read), is planned for the
future, and some room is left for it.
 
Discussion: https://www.postgresql.org/message-id/flat/-4019-bdb1-699e-@2ndquadrant.com

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Waiting for PostgreSQL 12 – REINDEX CONCURRENTLY

On 29th of March 2019, Peter Eisentraut committed patch:

REINDEX CONCURRENTLY 
 
This adds the CONCURRENTLY option to the REINDEX command.  A REINDEX
CONCURRENTLY on a specific index creates a new index (like CREATE
INDEX CONCURRENTLY), then renames the old index away and the new index
in place and adjusts the dependencies, and then drops the old
index (like DROP INDEX CONCURRENTLY).  The REINDEX command also has
the capability to run its other variants (TABLE, DATABASE) with the
CONCURRENTLY option (but not SYSTEM).
 
The reindexdb command gets the --concurrently option.
 
Author: Michael Paquier, Andreas Karlsson, Peter Eisentraut
 
Discussion: https://www.postgresql.org/message-id/flat/-956b-4478-45ed-%402ndquadrant.com#

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Migrating simple table to partitioned. How?

Recently someone asked, on irc, how to make table partitioned.

The thing is that it was supposed to be done with new partitioning, and not the old way.

The problem is that while we can create table that will be seen as partitioned – we can't alter table to become partitioned.

So. Is it possible?

Continue reading Migrating simple table to partitioned. How?

Waiting for PostgreSQL 12 – Partial implementation of SQL/JSON path language

On 16th of March 2019, Alexander Korotkov committed patch:

Partial implementation of SQL/JSON path language
 
SQL 2016 standards among other things contains set of SQL/JSON features for
JSON processing inside of relational database.  The core of SQL/JSON is JSON
path language, allowing access parts of JSON documents and make computations
over them.  This commit implements partial support JSON path language as
separate datatype called "jsonpath".  The implementation is partial because
it's lacking datetime support and suppression of numeric errors.  Missing
features will be added later by separate commits.
 
Support of SQL/JSON features requires implementation of separate nodes, and it
will be considered in subsequent patches.  This commit includes following
set of plain functions, allowing to execute jsonpath over jsonb values:
 
 * jsonb_path_exists(jsonb, jsonpath[, jsonb, bool]),
 * jsonb_path_match(jsonb, jsonpath[, jsonb, bool]),
 * jsonb_path_query(jsonb, jsonpath[, jsonb, bool]),
 * jsonb_path_query_array(jsonb, jsonpath[, jsonb, bool]).
 * jsonb_path_query_first(jsonb, jsonpath[, jsonb, bool]).
 
This commit also implements "jsonb @? jsonpath" and "jsonb @@ jsonpath", which
are wrappers over jsonpath_exists(jsonb, jsonpath) and jsonpath_predicate(jsonb,
jsonpath) correspondingly.  These operators will have an index support
(implemented in subsequent patches).
 
Catversion bumped, to add new functions and operators.
 
Code was written by Nikita Glukhov and Teodor Sigaev, revised by me.
Documentation was written by Oleg Bartunov and Liudmila Mantrova.  The work
was inspired by Oleg Bartunov.
 
Discussion: https://postgr.es/m/-b497-f39a-923d-%402ndQuadrant.com
Author: Nikita Glukhov, Teodor Sigaev, Alexander Korotkov, Oleg Bartunov, Liudmila Mantrova

Continue reading Waiting for PostgreSQL 12 – Partial implementation of SQL/JSON path language

why-upgrade updates

Recent change in layout of PG Docs broke my spider for why-upgrade.depesz.com.

Today got some time and decided to bite the bullet.

Fixed spider code, used it to get new changelog, and while I was at it, did couple of slight modifications of the site:

  • display count of all changes that are there in given upgrade path (in section header)
  • fixed a bug that caused empty Security fixes: box to display, even when there are no security fixes.
  • added metainfo page, which shows some stats about why-upgrade “database".

All in all, not big changes, but I hope you'll find it useful.

Waiting for PostgreSQL 12 – Allow user control of CTE materialization, and change the default behavior.

On 16th of February 2019, Tom Lane committed patch:

Allow user control of CTE materialization, and change the default behavior.
 
Historically we've always materialized the full output of a CTE query,
treating WITH as an optimization fence (so that, for example, restrictions
from the outer query cannot be pushed into it).  This is appropriate when
the CTE query is INSERT/UPDATE/DELETE, or is recursive; but when the CTE
query is non-recursive and side-effect-free, there's no hazard of changing
the query results by pushing restrictions down.
 
Another argument for materialization is that it can avoid duplicate
computation of an expensive WITH query --- but that only applies if
the WITH query is called more than once in the outer query.  Even then
it could still be a net loss, if each call has restrictions that
would allow just a small part of the WITH query to be computed.
 
Hence, let's change the behavior for WITH queries that are non-recursive
and side-effect-free.  By default, we will inline them into the outer
query (removing the optimization fence) if they are called just once.
If they are called more than once, we will keep the old behavior by
default, but the user can override this and force inlining by specifying
NOT MATERIALIZED.  Lastly, the user can force the old behavior by
specifying MATERIALIZED; this would mainly be useful when the query had
deliberately been employing WITH as an optimization fence to prevent a
poor choice of plan.
 
Andreas Karlsson, Andrew Gierth, David Fetter
 
Discussion: https://postgr.es/m/87sh48ffhb.fsf@news-spur.riddles.org.uk

Continue reading Waiting for PostgreSQL 12 – Allow user control of CTE materialization, and change the default behavior.

Converting list of integers into list of ranges

Yesterday someone on irc asked:

i've a query that returns sequential numbers with gaps (generate_series + join) and my question is: can is somehow construct ranges out of the returned values? sort of range_agg or something?

There was no further discussion, aside from me saying

sure you can. not trivial task, but possible.
you'd need window functions.

but it got me thinking …

Continue reading Converting list of integers into list of ranges

Waiting for PostgreSQL 12 – Allow COPY FROM to filter data using WHERE conditions

On 19th of January 2019, Tomas Vondra committed patch:

Allow COPY FROM to filter data using WHERE conditions
 
Extends the COPY FROM command with a WHERE condition, which allows doing
various types of filtering while importing the data (random sampling,
condition on a data column, etc.).  Until now such filtering required
either preprocessing of the input data, or importing all data and then
filtering in the database. COPY FROM ... WHERE is an easy-to-use and
low-overhead alternative for most simple cases.
 
Author: Surafel Temesgen
 
Discussion: https://www.postgresql.org/message-id/flat/CALAY4q_DdpWDuB5-Zyi-oTtO2uSk8pmy+dupiRe3AvAc++1imA@mail.gmail.com

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Waiting for PostgreSQL 12 – Add log_statement_sample_rate parameter

On 29th of November 2018, Alvaro Herrera committed patch:

Add log_statement_sample_rate parameter
 
This allows to set a lower log_min_duration_statement value without
incurring excessive log traffic (which reduces performance).  This can
be useful to analyze workloads with lots of short queries.
 
Author: Adrien Nayrat
 
Discussion: https://postgr.es/m/-ee1e-db9f-fa97-@anayrat.info

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