Join us on YugabyteDB Community Slack
Star us on
Get Started
Slack
GitHub
Get Started
v2.15 (preview) v2.12 (stable) v2.8 (earlier version) v2.6 (earlier version) v2.4 (earlier version) Unsupported versions
  • YUGABYTEDB CORE
    • Quick start
      • 1. Install YugabyteDB
      • 2. Create a local cluster
      • 3. Explore distributed SQL
      • 4. Build an application
        • Java
        • Node.js
        • Go
        • Python
        • Ruby
        • C#
        • PHP
        • C++
        • C
        • Scala
        • Rust
    • Explore
      • SQL features
        • SQL Feature Support
        • PostgreSQL Compatibility
        • Foreign Data Wrappers
        • Schemas and Tables
        • Data Types
        • Data Manipulation
        • Queries and Joins
        • Expressions and Operators
        • Stored Procedures
        • Triggers
        • Advanced features
          • Cursors
          • Table Partitioning
          • Views
          • Savepoints
          • Collations
        • Going beyond SQL
          • Follower reads
          • Tablespaces
        • PostgreSQL extensions
      • Fault tolerance
      • Horizontal scalability
        • Scaling Transactions
        • Sharding Data
      • Transactions
        • Distributed Transactions
        • Isolation Levels
        • Explicit Locking
      • Indexes and Constraints
        • Primary keys
        • Foreign keys
        • Secondary indexes
        • Unique indexes
        • Partial indexes
        • Expression indexes
        • Covering indexes
        • GIN indexes
        • Other constraints
      • JSON support
      • Multi-region deployments
        • Sync replication (3+ regions)
        • Async Replication (2+ regions)
        • Row-Level Geo-Partitioning
        • Read replicas
      • Query tuning
        • Introduction
        • Get query statistics using pg_stat_statements
        • Viewing live queries with pg_stat_activity
        • Analyzing queries with EXPLAIN
        • Optimizing YSQL queries using pg_hint_plan
      • Cluster management
        • Point-in-time recovery
      • Change data capture (CDC)
        • CDCSDK Server
        • Debezium connector
        • Java CDC console
      • Security
      • Observability
        • Prometheus Integration
        • Grafana Dashboard
    • Drivers and ORMs
      • Java
        • Connect an app
        • Use an ORM
      • Go
        • Go drivers
        • Go ORMs
        • Supported versions
      • C#
        • Connect an app
        • Use an ORM
      • NodeJS
        • NodeJS drivers
        • NodeJS ORMs
        • Supported Versions
      • Python
        • Python drivers
        • Python ORMs
        • Supported versions
      • Rust
        • Diesel ORM
    • Develop
      • Learn app development
        • 1. SQL vs NoSQL
        • 2. Data modeling
        • 3. Data types
        • 4. ACID transactions
        • 5. Aggregations
        • 6. Batch operations
        • 7. Date and time
        • 8. Strings and text
        • 9. TTL for data expiration
      • Real-world examples
        • E-Commerce app
        • IoT fleet management
      • Explore sample apps
      • Best practices
      • Quality of Service
        • Rate limiting connections
        • Write-heavy workloads
        • Transaction priorities
      • Cloud-native development
        • Codespaces
        • Gitpod
    • Migrate
      • YugabyteDB Voyager
        • Install
        • Migration steps
        • Performance
        • yb-voyager CLI
      • Manual import
        • Convert a PostgreSQL schema
        • Migrate a PostgreSQL application
        • Export PostgreSQL data
        • Prepare a cluster
        • Import PostgreSQL data
        • Verify migration
    • Deploy
      • Deployment checklist
      • Manual deployment
        • 1. System configuration
        • 2. Install software
        • 3. Start YB-Masters
        • 4. Start YB-TServers
        • 5. Verify deployment
      • Kubernetes
        • Single-zone
          • Open Source
          • Amazon EKS
          • Google Kubernetes Engine
          • Azure Kubernetes Service
        • Multi-zone
          • Amazon EKS
          • Google Kubernetes Engine
        • Multi-cluster
          • Google Kubernetes Engine
        • Best practices
        • Connect Clients
      • Docker
      • Public clouds
        • Amazon Web Services
        • Google Cloud Platform
        • Microsoft Azure
      • Multi-DC deployments
        • Three+ data center (3DC)
        • Asynchronous replication
        • Read replica clusters
    • Benchmark
      • TPC-C
      • sysbench
      • YCSB
      • Key-value workload
      • Large datasets
      • Scalability
        • Scaling queries
      • Resilience
        • Jepsen testing
      • Performance Troubleshooting
    • Secure
      • Security checklist
      • Enable authentication
        • Enable users
        • Configure client authentication
      • Authentication methods
        • Password authentication
        • LDAP authentication
        • Host-based authentication
        • Trust authentication
      • Role-based access control
        • Overview
        • Manage users and roles
        • Grant privileges
        • Row-level security
        • Column-level security
      • Encryption in transit
        • Create server certificates
        • Enable server-to-server encryption
        • Enable client-to-server encryption
        • Connect to clusters
        • TLS and authentication
      • Encryption at rest
      • Column-level encryption
      • Audit logging
        • Configure audit logging
        • Session-Level Audit Logging
        • Object-Level Audit Logging
      • Vulnerability disclosure policy
    • Manage
      • Backup and restore
        • Export and import
        • Distributed snapshots
        • Point-in-time recovery
      • Migrate data
        • Bulk import
        • Bulk export
      • Change cluster configuration
      • Diagnostics reporting
      • Upgrade a deployment
      • Grow cluster
    • Troubleshoot
      • Troubleshooting
      • Cluster level issues
        • YCQL connection issues
        • YEDIS connection Issues
        • Recover tserver/master
        • Replace a failed YB-TServer
        • Replace a failed YB-Master
        • Manual remote bootstrap when a majority of peers fail
        • Recover YB-TServer from crash loop
      • Node level issues
        • Check servers
        • Inspect logs
        • System statistics
        • Disk failure
        • Common error messages
    • Contribute
      • Core database
        • Contribution checklist
        • Build the source
        • Configure a CLion project
        • Run the tests
        • Coding style
      • Documentation
        • Docs checklist
        • Docs layout
        • Build the docs
          • Editor setup
        • Edit the docs
          • Docs page structure
          • Widgets and shortcodes
          • Syntax diagrams
        • Style guide
  • YUGABYTEDB ANYWHERE
    • Overview
      • Install
      • Configure
    • Install
      • Prerequisites
      • Prepare the environment
      • Install software
      • Prepare nodes
      • Uninstall software
    • Configure
      • Create admin user
      • Configure cloud providers
      • Configure backup target
      • Configure alerts
    • Create deployments
      • Multi-zone universe
      • Multi-region universe
      • Multi-cloud universe
      • Read replica cluster
      • Asynchronous replication
    • Manage deployments
      • Start and stop processes
      • Eliminate an unresponsive node
      • Recover a node
      • Enable high availability
      • Edit configuration flags
      • Edit a universe
      • Delete a universe
      • Configure instance tags
      • Upgrade YugabyteDB
      • Migrate to Helm 3
    • Back up universes
      • Configure backup storage
      • Back up universe data
      • Restore universe data
      • Schedule data backups
    • Security
      • Security checklist
      • Configure ports
      • LDAP authentication
      • Authorization
      • Create a KMS configuration
      • Enable encryption at rest
      • Enable encryption in transit
      • Network security
    • Alerts and monitoring
      • Alerts
      • Live Queries dashboard
      • Slow Queries dashboard
    • Troubleshoot
      • Install and upgrade issues
      • Cloud provider configuration issues
      • Universe issues
    • Administer
      • Back up YugabyteDB Anywhere
      • Configure authentication
    • Upgrade
      • Upgrade Kubernetes installation
      • Upgrade using Replicated
  • YUGABYTEDB MANAGED
    • Overview
    • Quick start
      • Create a Sandbox cluster
      • Connect to the cluster
      • Explore distributed SQL
      • Build an application
        • Before you begin
        • Java
        • Go
        • Python
        • Node.js
        • C
        • C++
        • C#
        • Ruby
        • Rust
        • PHP
    • Deploy clusters
      • Plan your cluster
      • Choose a topology
      • VPC network
        • Overview
        • VPCs
        • Peering connections
        • Create a VPC Network
      • Create your cluster
        • Sandbox
        • Single region
        • Replicate across regions
    • Secure clusters
      • IP allow lists
      • Database authorization
      • Add database users
      • Encryption in transit
      • Audit account activity
    • Connect to clusters
      • Cloud Shell
      • Client shell
      • Connect applications
    • Alerts and monitoring
      • Alerts
      • Performance metrics
      • Live queries
      • Slow queries
      • Performance Advisor
      • Cluster activity
    • Manage clusters
      • Scale and configure clusters
      • Backup and restore
      • Maintenance windows
      • Create extensions
    • Administration and billing
      • Manage account access
      • Manage API keys
      • Manage billing
      • Cluster costs
    • Example applications
      • Connect a Spring application
      • Connect a YCQL Java application
      • Hasura Cloud
      • Deploy a GraphQL application
    • Security architecture
      • Security architecture
      • Shared responsibility model
    • Troubleshoot
    • YugabyteDB Managed FAQ
    • What's new
  • INTEGRATIONS
    • Apache Kafka
    • Apache Spark
    • Camunda
    • Debezium
    • Django REST framework
    • Entity Framework
    • Flyway
    • GORM
    • Hasura
      • Application Development
      • Benchmarking
    • JanusGraph
    • KairosDB
    • Liquibase
    • Metabase
    • Presto
    • Prisma
    • Sequelize
    • Spring Framework
      • Spring Data YugabyteDB
      • Spring Data JPA
      • Spring Data Cassandra
    • SQLAlchemy
    • WSO2 Identity Server
    • YSQL Loader
    • YugabyteDB JDBC driver
  • REFERENCE
    • Architecture
      • Design goals
      • Key concepts
        • Universe
        • YB-TServer Service
        • YB-Master Service
      • Core functions
        • Universe creation
        • Table creation
        • Write IO path
        • Read IO path
        • High availability
      • Layered architecture
      • Query layer
        • Overview
      • DocDB transactions layer
        • Transactions overview
        • Transaction isolation levels
        • Explicit locking
        • Read Committed
        • Single-row transactions
        • Distributed transactions
        • Transactional IO path
      • DocDB sharding layer
        • Hash & range sharding
        • Tablet splitting
        • Colocated tables
      • DocDB replication layer
        • Replication
        • xCluster replication
        • Read replicas
        • Change data capture (CDC)
      • DocDB storage layer
        • Persistence
        • Performance
    • APIs
      • YSQL
        • The SQL language
          • SQL statements
            • ABORT
            • ALTER DATABASE
            • ALTER DEFAULT PRIVILEGES
            • ALTER DOMAIN
            • ALTER FOREIGN DATA WRAPPER
            • ALTER FOREIGN TABLE
            • ALTER GROUP
            • ALTER POLICY
            • ALTER ROLE
            • ALTER SEQUENCE
            • ALTER SERVER
            • ALTER TABLE
            • ALTER USER
            • ANALYZE
            • BEGIN
            • CALL
            • COMMENT
            • COMMIT
            • COPY
            • CREATE AGGREGATE
            • CREATE CAST
            • CREATE DATABASE
            • CREATE DOMAIN
            • CREATE EXTENSION
            • CREATE FOREIGN DATA WRAPPER
            • CREATE FOREIGN TABLE
            • CREATE FUNCTION
            • CREATE GROUP
            • CREATE INDEX
            • CREATE MATERIALIZED VIEW
            • CREATE OPERATOR
            • CREATE OPERATOR CLASS
            • CREATE POLICY
            • CREATE PROCEDURE
            • CREATE ROLE
            • CREATE RULE
            • CREATE SCHEMA
            • CREATE SEQUENCE
            • CREATE SERVER
            • CREATE TABLE
            • CREATE TABLE AS
            • CREATE TRIGGER
            • CREATE TYPE
            • CREATE USER
            • CREATE USER MAPPING
            • CREATE VIEW
            • DEALLOCATE
            • DELETE
            • DO
            • DROP AGGREGATE
            • DROP CAST
            • DROP DATABASE
            • DROP DOMAIN
            • DROP EXTENSION
            • DROP FOREIGN DATA WRAPPER
            • DROP FOREIGN TABLE
            • DROP FUNCTION
            • DROP GROUP
            • DROP MATERIALIZED VIEW
            • DROP OPERATOR
            • DROP OPERATOR CLASS
            • DROP OWNED
            • DROP POLICY
            • DROP PROCEDURE
            • DROP ROLE
            • DROP RULE
            • DROP SEQUENCE
            • DROP SERVER
            • DROP TABLE
            • DROP TRIGGER
            • DROP TYPE
            • DROP USER
            • END
            • EXECUTE
            • EXPLAIN
            • GRANT
            • IMPORT FOREIGN SCHEMA
            • INSERT
            • LOCK
            • PREPARE
            • REASSIGN OWNED
            • REFRESH MATERIALIZED VIEW
            • RELEASE SAVEPOINT
            • RESET
            • REVOKE
            • ROLLBACK
            • ROLLBACK TO SAVEPOINT
            • SAVEPOINT
            • SELECT
            • SET
            • SET CONSTRAINTS
            • SET ROLE
            • SET SESSION AUTHORIZATION
            • SET TRANSACTION
            • SHOW
            • SHOW TRANSACTION
            • TRUNCATE
            • UPDATE
            • VALUES
          • WITH clause
            • WITH clause—SQL syntax and semantics
            • recursive CTE
            • case study—traversing an employee hierarchy
            • traversing general graphs
              • graph representation
              • common code
              • undirected cyclic graph
              • directed cyclic graph
              • directed acyclic graph
              • rooted tree
              • Unique containing paths
              • Stress testing find_paths()
            • case study—Bacon Numbers from IMDb
              • Bacon numbers for synthetic data
              • Bacon numbers for IMDb data
        • Data types
          • Array
            • array[] constructor
            • Literals
              • Text typecasting and literals
              • Array of primitive values
              • Row
              • Array of rows
            • FOREACH loop (PL/pgSQL)
            • array of DOMAINs
            • Functions and operators
              • ANY and ALL
              • Array comparison
              • Array slice operator
              • Array concatenation
              • Array properties
              • array_agg(), unnest(), generate_subscripts()
              • array_fill()
              • array_position(), array_positions()
              • array_remove()
              • array_replace() / set value
              • array_to_string()
              • string_to_array()
          • Binary
          • Boolean
          • Character
          • Date and time
            • Conceptual background
            • Timezones and UTC offsets
              • Catalog views
              • Extended_timezone_names
                • Unrestricted full projection
                • Real timezones with DST
                • Real timezones no DST
                • Synthetic timezones no DST
              • Offset/timezone-sensitive operations
                • Timestamptz to/from timestamp conversion
                • Pure 'day' interval arithmetic
              • Four ways to specify offset
                • Name-resolution rules
                  • 1 case-insensitive resolution
                  • 2 ~names.abbrev never searched
                  • 3 'set timezone' string not resolved in ~abbrevs.abbrev
                  • 4 ~abbrevs.abbrev before ~names.name
                  • Helper functions
              • Syntax contexts for offset
              • Recommended practice
            • Typecasting between date-time and text-values
            • Semantics of the date-time data types
              • Date data type
              • Time data type
              • Plain timestamp and timestamptz
              • Interval data type
                • Interval representation
                  • Ad hoc examples
                  • Representation model
                • Interval value limits
                • Declaring intervals
                • Justify() and extract(epoch...)
                • Interval arithmetic
                  • Interval-interval comparison
                  • Interval-interval addition and subtraction
                  • Interval-number multiplication
                  • Moment-moment overloads of "-"
                  • Moment-interval overloads of "+" and "-"
                • Custom interval domains
                • Interval utility functions
            • Typecasting between date-time datatypes
            • Operators
              • Test comparison overloads
              • Test addition overloads
              • Test subtraction overloads
              • Test multiplication overloads
              • Test division overloads
            • General-purpose functions
              • Creating date-time values
              • Manipulating date-time values
              • Current date-time moment
              • Delaying execution
              • Miscellaneous
                • Function age()
                • Function extract() | date_part()
                • Implementations that model the overlaps operator
            • Formatting functions
            • Case study—SQL stopwatch
            • Download & install the date-time utilities
            • ToC
          • JSON
            • JSON literals
            • Primitive and compound data types
            • Code example conventions
            • Indexes and check constraints
            • Functions & operators
              • ::jsonb, ::json, ::text (typecast)
              • ->, ->>, #>, #>> (JSON subvalues)
              • - and #- (remove)
              • || (concatenation)
              • = (equality)
              • @> and <@ (containment)
              • ? and ?| and ?& (key or value existence)
              • array_to_json()
              • jsonb_agg()
              • jsonb_array_elements()
              • jsonb_array_elements_text()
              • jsonb_array_length()
              • jsonb_build_object()
              • jsonb_build_array()
              • jsonb_each()
              • jsonb_each_text()
              • jsonb_extract_path()
              • jsonb_extract_path_text() and json_extract_path_text()
              • jsonb_object()
              • jsonb_object_agg()
              • jsonb_object_keys()
              • jsonb_populate_record()
              • jsonb_populate_recordset()
              • jsonb_pretty()
              • jsonb_set() and jsonb_insert()
              • jsonb_strip_nulls()
              • jsonb_to_record()
              • jsonb_to_recordset()
              • jsonb_typeof()
              • row_to_json()
              • to_jsonb()
          • Money
          • Numeric
          • Range
          • Serial
          • UUID
        • Functions and operators
          • Aggregate functions
            • Informal functionality overview
            • Invocation syntax and semantics
            • grouping sets, rollup, cube
            • Per function signature and purpose
              • avg(), count(), max(), min(), sum()
              • array_agg(), string_agg(), jsonb_agg(), jsonb_object_agg()
              • bit_and(), bit_or(), bool_and(), bool_or()
              • variance(), var_pop(), var_samp(), stddev(), stddev_pop(), stddev_samp()
              • linear regression
                • covar_pop(), covar_samp(), corr()
                • regr_%()
              • mode(), percentile_disc(), percentile_cont()
              • rank(), dense_rank(), percent_rank(), cume_dist()
            • case study—percentile_cont() and the "68–95–99.7" rule
            • case study—linear regression on COVID data
              • Download the COVIDcast data
              • Ingest the COVIDcast data
                • Inspect the COVIDcast data
                • Copy the .csv files to staging tables
                • Check staged data conforms to the rules
                • Join the staged data into a single table
                • SQL scripts
                  • Create cr_staging_tables()
                  • Create cr_copy_from_scripts()
                  • Create assert_assumptions_ok()
                  • Create xform_to_covidcast_fb_survey_results()
                  • ingest-the-data.sql
              • Analyze the COVIDcast data
                • symptoms vs mask-wearing by day
                • Data for scatter-plot for 21-Oct-2020
                • Scatter-plot for 21-Oct-2020
                • SQL scripts
                  • analysis-queries.sql
                  • synthetic-data.sql
          • currval()
          • lastval()
          • nextval()
          • setval()
          • Window functions
            • Informal functionality overview
            • Invocation syntax and semantics
            • Per function signature and purpose
              • row_number(), rank() and dense_rank()
              • percent_rank(), cume_dist() and ntile()
              • first_value(), nth_value(), last_value()
              • lag(), lead()
              • Tables for the code examples
                • table t1
                • table t2
                • table t3
                • table t4
            • case study—analyzing a normal distribution
              • Bucket allocation scheme
              • do_clean_start.sql
              • cr_show_t4.sql
              • cr_dp_views.sql
              • cr_int_views.sql
              • cr_pr_cd_equality_report.sql
              • cr_bucket_using_width_bucket.sql
              • cr_bucket_dedicated_code.sql
              • do_assert_bucket_ok
              • cr_histogram.sql
              • cr_do_ntile.sql
              • cr_do_percent_rank.sql
              • cr_do_cume_dist.sql
              • do_populate_results.sql
              • do_report_results.sql
              • do_compare_dp_results.sql
              • do_demo.sql
              • Reports
                • Histogram report
                • dp-results
                • compare-dp-results
                • int-results
          • yb_hash_code()
        • Keywords
        • Reserved names
      • YCQL
        • ALTER KEYSPACE
        • ALTER ROLE
        • ALTER TABLE
        • CREATE INDEX
        • CREATE KEYSPACE
        • CREATE ROLE
        • CREATE TABLE
        • CREATE TYPE
        • DROP INDEX
        • DROP KEYSPACE
        • DROP ROLE
        • DROP TABLE
        • DROP TYPE
        • GRANT PERMISSION
        • GRANT ROLE
        • REVOKE PERMISSION
        • REVOKE ROLE
        • USE
        • INSERT
        • SELECT
        • EXPLAIN
        • UPDATE
        • DELETE
        • TRANSACTION
        • TRUNCATE
        • Simple expressions
        • Subscripted expressions
        • Function call
        • Operators
        • BLOB
        • BOOLEAN
        • Collection
        • FROZEN
        • INET
        • Integer and counter
        • Non-integer
        • TEXT
        • DATE, TIME, and TIMESTAMP
        • UUID and TIMEUUID
        • JSONB
        • Date and time
        • BATCH
    • CLIs
      • yb-ctl
      • yb-docker-ctl
      • ysqlsh
      • ycqlsh
      • yb-admin
      • yb-ts-cli
      • ysql_dump
      • ysql_dumpall
    • Configuration
      • yb-tserver
      • yb-master
      • yugabyted
      • Default ports
    • Drivers and ORMs
      • JDBC Drivers
      • C# Drivers
      • Go Drivers
      • Python Drivers
      • Client drivers for YSQL
      • Client drivers for YCQL
    • Connectors
      • Kafka Connect YugabyteDB
    • Third party tools
      • pgAdmin
      • Apache Superset
      • Arctype
      • DBeaver
      • TablePlus
      • DbSchema
      • SQL Workbench/J
      • Cassandra Workbench
    • Sample datasets
      • Chinook
      • Northwind
      • PgExercises
      • SportsDB
      • Retail Analytics
  • RELEASES
    • Releases overview
      • v2.15 series (preview)
      • v2.13 series
      • v2.12 series (stable)
      • v2.11 series
      • v2.9 series
      • v2.8 series
      • v2.7 series
      • v2.6 series
      • v2.5 series
      • v2.4 series
      • v2.3 series
      • v2.2 series
      • v2.1 series
      • v2.0 series
      • v1.3 series
      • v1.2 series
    • Release versioning
  • FAQ
    • Comparisons
      • Amazon Aurora
      • Google Cloud Spanner
      • CockroachDB
      • TiDB
      • Vitess
      • MongoDB
      • FoundationDB
      • Amazon DynamoDB
      • Azure Cosmos DB
      • Apache Cassandra
      • PostgreSQL
      • Redis in-memory store
      • Apache HBase
    • General FAQ
    • Operations FAQ
    • API compatibility FAQ
    • YugabyteDB Anywhere FAQ
  • MISC
    • YEDIS
      • Quick start
      • Develop
        • Build an application
        • C#
        • C++
        • Go
        • Java
        • NodeJS
        • Python
      • API reference
        • APPEND
        • AUTH
        • CONFIG
        • CREATEDB
        • DELETEDB
        • LISTDB
        • SELECT
        • DEL
        • ECHO
        • EXISTS
        • EXPIRE
        • EXPIREAT
        • FLUSHALL
        • FLUSHDB
        • GET
        • GETRANGE
        • GETSET
        • HDEL
        • HEXISTS
        • HGET
        • HGETALL
        • HINCRBY
        • HKEYS
        • HLEN
        • HMGET
        • HMSET
        • HSET
        • HSTRLEN
        • HVALS
        • INCR
        • INCRBY
        • KEYS
        • MONITOR
        • PEXPIRE
        • PEXPIREAT
        • PTTL
        • ROLE
        • SADD
        • SCARD
        • RENAME
        • SET
        • SETEX
        • PSETEX
        • SETRANGE
        • SISMEMBER
        • SMEMBERS
        • SREM
        • STRLEN
        • ZRANGE
        • TSADD
        • TSCARD
        • TSGET
        • TSLASTN
        • TSRANGEBYTIME
        • TSREM
        • TSREVRANGEBYTIME
        • TTL
        • ZADD
        • ZCARD
        • ZRANGEBYSCORE
        • ZREM
        • ZREVRANGE
        • ZSCORE
        • PUBSUB
        • PUBLISH
        • SUBSCRIBE
        • UNSUBSCRIBE
        • PSUBSCRIBE
        • PUNSUBSCRIBE
    • Legal
      • Third party software
> Architecture > DocDB transactions layer >

Read Committed isolation level Beta

Report a doc issue Suggest new content Contributor guide
  • Semantics
    • SELECT (without explicit row locking)
    • UPDATE, DELETE, SELECT FOR UPDATE, FOR SHARE, FOR NO KEY UPDATE, FOR KEY SHARE
    • INSERT
  • Usage
  • Examples
    • Avoid deadlocks in Read Committed transactions
    • SELECT behavior without explicit locking
    • UPDATE behavior
    • SELECT FOR UPDATE behavior
    • INSERT behavior
  • Cross feature interaction
  • Limitations
  • Noteworthy Considerations
    • Performance tuning

Read Committed is one of the three isolation levels in PostgreSQL, and also its default. A unique property of this isolation level is that clients don't need retry logic for serialization errors (40001) in applications when using this isolation level.

The other two isolation levels (Serializable and Repeatable Read) require apps to have retry logic for serialization errors. Read Committed in PostgreSQL works around conflicts by allowing single statements to work on an inconsistent snapshot (in other words, non-conflicting rows are read as of the statement's snapshot, but conflict resolution is done by reading and attempting re-execution/ locking on the latest version of the row).

YSQL supports the Read Committed isolation level, and its behavior is the same as that of PostgreSQL's Read Committed level (section 13.2.1).

Semantics

YSQL requirement

In addition to the requirements that follow, there is another YSQL specific requirement: ensure that external clients don't face kReadRestart errors.

To support the Read Committed isolation level in YSQL with the same semantics as PostgreSQL, the following requirements apply:

SELECT (without explicit row locking)

  1. New read point is chosen at statement start that includes anything that committed before the query began.
  2. Data from updates by previous statements in the same transaction is visible.

UPDATE, DELETE, SELECT FOR UPDATE, FOR SHARE, FOR NO KEY UPDATE, FOR KEY SHARE

(The last two are not mentioned in the PostgreSQL documentation, but the same behavior is seen for these as below.)

  1. New read point is chosen at statement start that includes anything that committed before the query began.
  2. If the row of interest:
    • is being updated (or deleted) by other transactions in a conflicting way (the statement's read time falls within the read time of other transactions to current time), wait for them to commit or rollback, then perform recheck steps (see below).
    • has been updated (or deleted) by other transactions in a conflicting way (a statement's read time falls within the read time to commit time of other transactions), perform recheck steps.
    • has been locked by other transactions in a conflicting way, wait for them to commit or rollback, then perform recheck steps.

The recheck steps are as follows:

  1. If a row is deleted, ignore it.
  2. Apply update/acquire lock on updated version of row if where clause evaluates to true on the updated version of row. (Note that the updated version of a row could have a different pk as well; this implies that PostgreSQL follows the chain of updates for a row even across primary key changes).

INSERT

  1. ON CONFLICT DO UPDATE: if a conflict occurs, wait for the conflicting transaction to commit or rollback.
    1. On rollback, proceed as usual.
    2. On commit, modify the new version of row.
  2. ON CONFLICT DO NOTHING: do nothing if a conflict occurs.

Usage

By setting the tserver gflag yb_enable_read_committed_isolation=true, the Read Committed isolation in YSQL will actually map to the Read Committed implementation in DocDB. If set to false, it will have the earlier behavior of mapping Read Committed to REPEATABLE READ.

The following ways can be used to start a Read Committed transaction after setting the gflag:

  1. START TRANSACTION isolation level read committed [read write | read only];
  2. BEGIN [TRANSACTION] isolation level read committed [read write | read only];
  3. BEGIN [TRANSACTION]; SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
  4. BEGIN [TRANSACTION]; SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL READ COMMITTED;

Read Committed on YSQL will have pessimistic locking behavior; in other words, a Read Committed transaction will wait for other Read Committed transactions to commit or rollback in case of a conflict. Two or more transactions could be waiting on each other in a cycle. Hence, to avoid a deadlock, be sure to configure a statement timeout (by setting the statement_timeout parameter in ysql_pg_conf_csv tserver gflag on cluster startup). Statement timeouts will help avoid deadlocks (see the first example).

Examples

Start by setting up the table you'll use in all of the examples in this section.

create table test (k int primary key, v int);

Avoid deadlocks in Read Committed transactions

You can avoid deadlocks in Read Committed transactions by relying on statement timeout.

truncate table test;
insert into test values (1, 5);
insert into test values (2, 5);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
set statement_timeout=2000;
update test set v=5 where k=1;
UPDATE 1
update test set v=5 where k=2;
UPDATE 1
update test set v=5 where k=2;
(waits)
update test set v=5 where k=1;
ERROR:  cancelling statement due to statement timeout
rollback;
UPDATE 1
commit;

SELECT behavior without explicit locking

truncate table test;
insert into test values (1, 5);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
select * from test where v=5;
 k | v
---+---
 1 | 5
(1 row)
insert into test values (2, 5);
INSERT 0 1
select * from test where v=5;
 k | v
---+---
 1 | 5
(1 row)
insert into test values (3, 5);
INSERT 0 1
select * from test where v=5;
 k | v
---+---
 1 | 5
 3 | 5
(2 rows)
commit;
select * from test where v=5;
 k | v
---+---
 1 | 5
 2 | 5
 3 | 5
(3 rows)
commit;

UPDATE behavior

truncate table test;
insert into test values (0, 5), (1, 5), (2, 5), (3, 5), (4, 1);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
insert into test values (5, 5);
INSERT 0 1
update test set v=10 where k=4;
UPDATE 1
delete from test where k=3;
DELETE 1
update test set v=10 where k=2;
UPDATE 1
update test set v=1 where k=1;
UPDATE 1
update test set k=10 where k=0;
UPDATE 1
update test set v=100 where v>=5;
(waits)
commit;
UPDATE 4
select * from test;
 k  |  v
----+-----
  5 | 100
  1 |   1
 10 | 100
  4 | 100
  2 | 100
(5 rows)
commit;

SELECT FOR UPDATE behavior

truncate table test;
insert into test values (0, 5), (1, 5), (2, 5), (3, 5), (4, 1);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
insert into test values (5, 5);
INSERT 0 1
update test set v=10 where k=4;
UPDATE 1
delete from test where k=3;
DELETE 1
update test set v=10 where k=2;
UPDATE 1
update test set v=1 where k=1;
UPDATE 1
update test set k=10 where k=0;
UPDATE 1
select * from test where v>=5 for update;
(waits)
commit;
 k  | v
----+----
  5 |  5
 10 |  5
  4 | 10
  2 | 10
(4 rows)
commit;

INSERT behavior

Insert a new key that is also just changed by another transaction

truncate table test;
insert into test values (1, 1);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
update test set k=2 where k=1;
UPDATE 1
insert into test values (2, 1);
(waits)
commit;
ERROR:  duplicate key value violates unique constraint "test_pkey"
rollback;

Same as previous, but with ON CONFLICT

truncate table test;
insert into test values (1, 1);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
update test set k=2 where k=1;
UPDATE 1
insert into test values (2, 1) on conflict (k) do update set v=100;
(waits)
commit;
INSERT 0 1
select * from test;
 k |  v
---+-----
 2 | 100
(1 row)
commit;

INSERT old key that is removed by other transaction

truncate table test;
insert into test values (1, 1);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
update test set k=2 where k=1;
UPDATE 1
insert into test values (1, 1);
(waits)
commit;
INSERT 0 1
select * from test;
 k | v
---+---
 1 | 1
 2 | 1
(2 rows)
commit;

Same as previous, but with ON CONFLICT

truncate table test;
insert into test values (1, 1);
Client 1 Client 2
begin transaction isolation level read committed;
begin transaction isolation level read committed;
update test set k=2 where k=1;
UPDATE 1
insert into test values (1, 1) on conflict (k) do update set v=100;
(waits)
commit;
INSERT 0 1
select * from test;
 k |  v
---+-----
 1 |  1
 2 |  1
(2 rows)
commit;

Cross feature interaction

This feature interacts with the following features:

  1. Follower reads (integration in progress): When follower reads is turned on, the read point for each statement in a Read Committed transaction will be picked as Now() - yb_follower_read_staleness_ms (if the transaction/statement is known to be explicitly/ implicitly read only).

  2. Pessimistic locking: Read Committed has a dependency on pessimistic locking to fully work. To be precise, on facing a conflict, a transaction has to wait for the conflicting transaction to rollback/commit. Pessimistic locking behavior can be seen for Read Committed. An optimized version of pessimistic locking will come in near future, which will give better performance and will also work for REPEATABLE READ and SERIALIZABLE isolation levels. The optimized version will also help detect deadlocks proactively instead of relying on statement timeouts for deadlock avoidance (see example 1).

Limitations

Work is in progress to remove these limitations:

  1. Read Committed semantics ensure that the client doesn't face conflict and read restart errors. YSQL maintains these semantics as long as a statement's output doesn't exceed ysql_output_buffer_size (a gflag with a default of 256KB). If this condition is not met, YSQL will resort to optimistic locking for that statement.

  2. PostgreSQL requires the following as mentioned in its docs: "STABLE and IMMUTABLE functions use a snapshot established as of the start of the calling query, whereas VOLATILE functions obtain a fresh snapshot at the start of each query they execute." YSQL uses a single snapshot for the whole procedure instead of one for each statement in the procedure.

Noteworthy Considerations

This isolation level allows both phantom and non-repeatable reads (as in this example).

Adding this new isolation level doesn't affect the performance of existing isolation levels.

Performance tuning

If a statement in the Read Committed isolation level faces a conflict, it will be retried with exponential backoff till the statement times out. The following parameters control the backoff:

  • retry_max_backoff is the maximum backoff in milliseconds between retries.
  • retry_min_backoff is the minimum backoff in milliseconds between retries.
  • retry_backoff_multiplier is the multiplier used to calculate the next retry backoff.

You can set these parameters on a per-session basis, or in the ysql_pg_conf_csv tserver gflag on cluster startup.

After the optimized version of pessimistic locking (as described in Cross-feature interaction) is completed, there won't be a need to hand tune these parameters for performance. Statements will restart only when all conflicting transactions have committed or rolled back, instead of retrying with an exponential backoff.

  • Semantics
    • SELECT (without explicit row locking)
    • UPDATE, DELETE, SELECT FOR UPDATE, FOR SHARE, FOR NO KEY UPDATE, FOR KEY SHARE
    • INSERT
  • Usage
  • Examples
    • Avoid deadlocks in Read Committed transactions
    • SELECT behavior without explicit locking
    • UPDATE behavior
    • SELECT FOR UPDATE behavior
    • INSERT behavior
  • Cross feature interaction
  • Limitations
  • Noteworthy Considerations
    • Performance tuning
Ask our community
  • Slack
  • Github
  • Forum
  • StackOverflow
Yugabyte
Contact Us
Copyright © 2017-2022 Yugabyte, Inc. All rights reserved.