Blog , p.3

January 12, 2021   •   News
Postgres Pro Standard 13.1.1 was released by the Postgres Professional team on December 18, 2020. This release is based on the corresponding version of the upstream PostgreSQL DBMS and includes a number of enhancements listed in this announcement.
December 18, 2020   •   PostgreSQL
Just to remind you, we already discussed issues related to isolation , made a digression regarding low-level data structure , and then explored row versions and observed how data snapshots are obtained from row versions. Now we will proceed to two closely connected problems: in-page vacuum и HOT updates . Both techniques can be referred to optimizations; they are important, but virtually not covered in the documentation. In-page vacuum during regular updates When accessing a page for either an update or read, if PostgreSQL understands that the page is running out of space, it can do a fast in-page vacuum. This happens in either of the cases: A previous update in this page did not find enough space to allocate a new row version in the same page. Such a situation is remembered in the page header, and next time the page is vacuumed. The page is more than fillfactor percent full. In this case, vacuum is performed right away without putting off till next time.
December 7, 2020   •   PostgreSQL

Hacking PostgreSQL 13 Webinar: Questions & Answers

To get things done before 2020 is over, I decided to publish my blog post with answers to your questions from our webinar on PostgreSQL 13 that we hosted in October. I’m still looking into release 13 and will soon give another talk on PG13 monitoring-related features that became available in this version of PostgreSQL. For now, let’s go through the webinar questions.
December 4, 2020   •   PostgreSQL
After having discussed isolation problems and having made a digression regarding the low-level data structure , last time we explored row versions and observed how different operations changed tuple header fields. Now we will look at how consistent data snapshots are obtained from tuples. What is a data snapshot? Data pages can physically contain several versions of the same row. But each transaction must see only one (or none) version of each row, so that all of them make up a consistent picture of the data (in the sense of ACID) as of a certain point in time. Isolation in PosgreSQL is based on snapshots: each transaction works with its own data snapshot, which "contains" data that were committed before the moment the snapshot was created and does not "contain" data that were not committed by that moment yet. We've already seen that although the resulting isolation appears stricter than required by the standard, it still has anomalies.
December 3, 2020   •   PostgreSQL

JSON In PostgreSQL: Questions and Answers

This blog post is written as a follow-up for the “Roadmap for JSON in PostgreSQL” webinar with Oleg Bartunov that we hosted in September. It took us a while to finally tackle these questions, but the proverb says ‘better late than never’.
November 24, 2020   •   PostgreSQL

PostgreSQL monitoring using Zabbix Agent 2: easy and extensible

Last year, the popular monitoring service Zabbix introduced Agent 2, designed to reduce the number of TCP connections and provide convenient extendability through plugins in Golang. In this article, I will discuss the main features of Zabbix Agent 2 plugin for monitoring PostgreSQL,  give tips on how to configure it and explain by example how to customize the plugin.
November 20, 2020   •   PostgreSQL
Well, we've already discussed isolation and made a digression regarding the low-level data structure . And we've finally reached the most fascinating thing, that is, row versions (tuples). Tuple header As already mentioned, several versions of each row can be simultaneously available in the database. And we need to somehow distinguish one version from another one. To this end, each version is labeled with its effective "time" ( xmin ) and expiration "time" ( xmax ). Quotation marks denote that a special incrementing counter is used rather than the time itself. And this counter is the transaction identifier . (As usual, in reality this is more complicated: the transaction ID cannot always increment due to a limited bit depth of the counter. But we will explore more details of this when our discussion reaches freezing.) When a row is created, the value of xmin is set equal to the ID of the transaction that performed the INSERT command, while xmax is not filled in. When a row is deleted, the xmax value of the current version is labeled with the ID of the transaction that performed DELETE. An UPDATE command actually performs two subsequent operations: DELETE and INSERT. In the current version of the row, xmax is set equal to the ID of the transaction that performed UPDATE. Then a new version of the same row is created, in which the value of xmin is the same as xmax of the previous version.
November 5, 2020   •   PostgreSQL
Last time we talked about data consistency, looked at the difference between levels of transaction isolation from the point of view of the user and figured out why this is important to know. Now we are starting to explore how PostgreSQL implements snapshot isolation and multiversion concurrency. In this article, we will look at how data is physically laid out in files and pages. This takes us away from discussing isolation, but such a digression is necessary to understand what follows. We will need to figure out how the data storage is organized at a low level. Relations If you look inside tables and indexes, it turns out that they are organized in a similar way. Both are database objects that contain some data consisting of rows. There is no doubt that a table consists of rows, but this is less obvious for an index. However, imagine a B-tree: it consists of nodes that contain indexed values and references to other nodes or table rows. It's these nodes that can be considered index rows, and in fact, they are. Actually, a few more objects are organized in a similar way: sequences (essentially single-row tables) and materialized views (essentially, tables that remember the query). And there are also regular views, which do not store data themselves, but are in all other senses similar to tables. All these objects in PostgreSQL are called the common word relation . This word is extremely improper because it is a term from the relational theory. You can draw a parallel between a relation and a table (view), but certainly not between a relation and an index. But it just so happened: the academic origin of PostgreSQL manifests itself. It seems to me that it's tables and views that were called so first, and the rest swelled over time.
October 30, 2020   •   PostgreSQL
With this article I start a series about the internal structure of PostgreSQL. The material will be based on our training courses on database administration that Pavel Luzanov and I are creating. Not everyone likes to watch video (I definitely do not), and reading slides, even with comments, is no good at all. We strongly recommend you to get familiar with our 2-Day Introduction to PostgreSQL 11 . Of course, the articles will not be exactly the same as the content of the courses. I will talk only about how everything is organized, omitting the administration itself, but I will try to do it in more detail and more thoroughly. And I believe that the knowledge like this is as useful to an application developer as it is to an administrator. I will target those who already have some experience in using PostgreSQL and at least in general understand what is what. The text will be too difficult for beginners. For example, I will not say a word about how to install PostgreSQL and run psql. The stuff in question does not vary much from version to version, but I will use PostgreSQL 11. The first series deals with issues related to isolation and multiversion concurrency, and the plan of the series is as follows: Isolation as understood by the standard and PostgreSQL (this article). Forks, files, pages — what is happening at the physical level. Row versions, virtual transactions and subtransactions. Data snapshots and the visibility of row versions; the event horizon. In-page vacuum and HOT updates. Normal vacuum. Autovacuum. Transaction id wraparound and freezing. Off we go!
October 26, 2020   •   PostgreSQL

What is Baked in the Baker's Dozen?

On April 8, PostgreSQL feature freeze took place, so only features committed earlier will get into version PostgreSQL 13. Probably, this version can hardly be considered revolutionary, since it has no conceptual changes. Some of critical patches were late to get into it, such as Table and Functions for the JSON/SQL standard, which had been desirable to be part of PostgreSQL 12, along with the JSONPath patch; plug-in warehouses did not appear either — only the interface is being finalized. The list of improvements is still impressive. We prepared a pretty complete overview of the patches included in the Baker's Dozen.
October 26, 2020   •   PostgreSQL

Parallelism in PostgreSQL: treatment of trees and conscience

  Database scaling is a continually coming future. DBMS get improved and better scaled on hardware platforms, while the hardware platforms themselves increase the performance, number of cores, and memory - Achilles is trying to catch up with the turtle, but has not caught up yet. The database scaling challenge manifests itself in all its magnitude.   Postgres Professional had to face the scaling problem not only theoretically, but also in practice: through their customers. Even more than once. It's one of these real-life cases that this article will discuss.   Many thanks to Elena Indrupskaya for the translation. Russian version is here .
October 9, 2020   •   PostgreSQL

Indexes in PostgreSQL — 10 (Bloom)

In the previous articles we discussed PostgreSQL indexing engine and the interface of access methods , as well as B-trees , GiST , SP-GiST , GIN , RUM , and BRIN . But we still need to look at Bloom indexes. Bloom General concept A classical Bloom filter is a data structure that enables us to quickly check membership of an element in a set. The filter is highly compact, but allows false positives: it can mistakenly consider an element to be a member of a set (false positive), but it is not permitted to consider an element of a set not to be a member (false negative). The filter is an array of m bits (also called a signature ) that is initially filled with zeros. k different hash functions are chosen that map any element of the set to k bits of the signature. To add an element to the set, we need to set each of these bits in the signature to one. Consequently, if all the bits corresponding to an element are set to one, the element can be a member of the set, but if at least one bit equals zero, the element is not in the set for sure. In the case of a DBMS, we actually have N separate filters built for each index row. As a rule, several fields are included in the index, and it's values of these fields that compose the set of elements for each row. By choosing the length of the signature m , we can find a trade-off between the index size and the probability of false positives. The application area for Bloom index is large, considerably "wide" tables to be queried using filters on each of the fields. This access method, like BRIN, can be regarded as an accelerator of sequential scan: all the matches found by the index must be rechecked with the table, but there is a chance to avoid considering most of the rows at all.
October 1, 2020   •   PostgreSQL

Indexes in PostgreSQL — 9 (BRIN)

In the previous articles we discussed PostgreSQL indexing engine , the interface of access methods , and the following methods: B-trees , GiST , SP-GiST , GIN , and RUM . The topic of this article is BRIN indexes. BRIN General concept Unlike indexes with which we've already got acquainted, the idea of BRIN is to avoid looking through definitely unsuited rows rather than quickly find the matching ones. This is always an inaccurate index: it does not contain TIDs of table rows at all. Simplistically, BRIN works fine for columns where values correlate with their physical location in the table. In other words, if a query without ORDER BY clause returns the column values virtually in the increasing or decreasing order (and there are no indexes on that column). This access method was created in scope of Axle , the European project for extremely large analytical databases, with an eye on tables that are several terabyte or dozens of terabytes large. An important feature of BRIN that enables us to create indexes on such tables is a small size and minimal overhead costs of maintenance. This works as follows. The table is split into ranges that are several pages large (or several blocks large, which is the same) - hence the name: Block Range Index, BRIN. The index stores summary information on the data in each range. As a rule, this is the minimal and maximal values, but it happens to be different, as shown further. Assume that a query is performed that contains the condition for a column; if the sought values do not get into the interval, the whole range can be skipped; but if they do get, all rows in all blocks will have to be looked through to choose the matching ones among them. It will not be a mistake to treat BRIN not as an index, but as an accelerator of sequential scan. We can regard BRIN as an alternative to partitioning if we consider each range as a "virtual" partition. Now let's discuss the structure of the index in more detail.
September 24, 2020   •   PostgreSQL

Indexes in PostgreSQL — 8 (RUM)

We have already discussed PostgreSQL indexing engine , the interface of access methods , and main access methods, such as: hash indexes , B-trees , GiST , SP-GiST , and GIN . In this article, we will watch how gin turns into rum. RUM Although the authors claim that gin is a powerful genie, the theme of drinks has eventually won: next-generation GIN has been called RUM. This access method expands the concept that underlies GIN and enables us to perform full-text search even faster. In this series of articles, this is the only method that is not included in a standard PostgreSQL delivery and is an external extension. Several installation options are available for it: Take "yum" or "apt" package from the PGDG repository . For example, if you installed PostgreSQL from "postgresql-10" package, also install "postgresql-10-rum". Build from source code on github and install on your own (the instruction is there as well). Use as a part of Postgres Pro Enterprise (or at least read the documentation from there). Limitations of GIN What limitations of GIN does RUM enable us to transcend? First, "tsvector" data type contains not only lexemes, but also information on their positions inside the document. As we observed last time , GIN index does not store this information. For this reason, operations to search for phrases, which appeared in version 9.6, are supported by GIN index inefficiently and have to access the original data for recheck. Second, search systems usually return the results sorted by relevance (whatever that means). We can use ranking functions "ts_rank" and "ts_rank_cd" to this end, but they have to be computed for each row of the result, which is certainly slow. To a first approximation, RUM access method can be considered as GIN that additionally stores position information and can return the results in a needed order (like GiST can return nearest neighbors). Let's move step by step.