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Questions tagged [mvcc]

MVCC - Multi Version Concurrency Control

MVCC is a method of concurrency control, typically used in databases and transactional memories. Each transaction sees a snapshot of database when it started and its changes are only seen when transaction is committed.This method provides more concurrency them locks. Use all popular databases like Oracle, PostGres, InnoDB of MySQL etc.

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What data storage systems allow queries about the state of the system in the past?

I'm designing a system where questions like "what was the sum of all records matching certain criteria, at a certain time" are important. Multi-value concurrency control (MVCC) seems like the way to go here, since it keeps an audit trail…
NeilK
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How Indices Cope with MVCC?

Greetings Overflowers, To my understanding (and I hope I'm not right) changes to indices cannot be MVCCed. I'm wondering if this is also true with big records as copies can be costly. Since records are accessed via indices (usually), how MVCC can…
geeko
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How PostgreSQL index deals with MVCC

In PostgreSQL every update of tuple creates new tuple version. So for some period of time there can be lot of versions of same tuple and different transactions can see different version of tuple (using visibility rules) Index is updated before…
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Is there a way to execute an SQL merge atomically?

I am looking for a way to connect a tuple of values with a random UUID in a non-locking manner and without the potential to fail a transaction due to concurrency constraints. The table I need to edit contains several values that should be described…
Rafael Winterhalter
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does Firebird defrag? If so, like a clustered index?

I've seen a few (literally, only a few) links and nothing in the documentation that talks about clustering with Firebird, that it can be done. Then, I shot for the moon on this question CLUSTER command for Firebird?, but answerer told me that…
user1382306
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Mysql InnoDB row locking from read

I have read mysql documentation and maybe I just don't understand it all. What I'm looking for is the row locking from reading for other sessions. While the row is locked I would like the other sessions to wait until the lock is released. I don't…
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Why PostgreSQL indexes do not contain visibility information?

I know that the physical storage in PostgreSQL looks like: heap table: index: So Index-Only Scan need the help of Visibility Map. My…
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PostgreSQL - MVCC (multi-version concurrency control) - When is actual lock acquired?

As per my understanding, postgres uses two additional fields Xmin and Xmax to implement mvcc, Lets say we have Employee table with id and name columns. Below are some crud operations and how they are working concurrently(considering isolation level…
user10916892
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Does PostgreSQL transaction id (xmin) appears in committed version sequentially?

Due to PostgreSQL docs https://www.postgresql.org/docs/current/ddl-system-columns.html xmin The identity (transaction ID) of the inserting transaction for this row version (A row version is an individual state of a row; each update of a row creates…
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Why does compiling a VCC .sln run in the background with no stdout?

I'm trying to compile a project from the command line, like this: devenv.exe myproj.sln /build release It looks like the code compiles well, but that's not all I need: I want to be able to capture the output (e.g. warnings, errors) from the…
Salim Fadhley
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How to avoid data fragmentation in an "INSERT once, UPDATE once" table?

I have large number of tables that are "INSERT once", and then read-only after that. ie: After the initial INSERT of a record, there are never any UPDATE or DELETE statements. Due to this, the data fragmentation on disk for the tables is…
Russ
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Database for long running transactions with huge updates

I build a tool for data extraction and transformation. Typical use case - transactionally processing lots of data. Numbers are - about 10sec - 5min duration, 200-10000 row updated (long duration caused not by the database itself but by outside…
Alex Craft
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Do MVCC databases see inserted rows in mid-transaction?

Does MVCC database isolation mode allow in-progress transactions to see rows inserted (and committed) by other transactions? For example, given: Table names[id BIGINT NOT NULL, name VARCHAR(30), PRIMARY KEY(id), UNIQUE(name)] Transactions T1 and…
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A call to a VOLATILE function breaks atomicity of a SELECT statement

Given (PostgreSQL 15.3): create table test ( test_id int2, test_val int4, primary key (test_id) ); If these two transactions are run in parallel at read committed isolation level: -- Transaction 1 insert into test (select 1, 1 from…
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How to reveal old versions of a row in PostgreSQL?

I know that Postgres uses MVCC and stores older versions of rows on disk. I would like to access (read-only) some old, discarded versions of a concrete row. How can I do it?
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