postgres partitioning vs sharding

Partitioning is an important subject to cover separate from sharding. Declarative partitioning in PostgreSQL 10. Jobin Augustine is a PostgreSQL expert and Open Source advocate and has more than 19 years of working experience as consultant, architect, administrator, writer, and trainer in PostgreSQL, Oracle and other database technologies. This method of filtering can avoid a full table scan and only scan a smaller subset of data. might pull in lots of rows from a foreign table it might slow things down. Defining your partition key (also called a ‘shard key’ or 'distribution key’) Sharding at the core is splitting your data up to where it resides in smaller chunks, spread across distinct separate buckets. First, let’s create the physical partition table on box2: And then create the partition on your server, as a foreign table: You can now insert and query from your own server: There you have it! Each partition has the same schema and columns, but also entirely different rows. Background.   •   However, you write: “It only ever makes sense to shard if the nature of the queries involving the target table(s) is such that distributed processing will be the norm […] Due to the distributed nature of sharding such queries will necessarily perform worse if compared to having them all hosted on the same server.” While I fully understand your point, I wonder why it shouldn’t be beneficial to have less data on each shard. The basis for this is in PostgreSQL’s Foreign Data Wrapper (FDW) support, which has been a part of the core of PostgreSQL for a long time. Sharding makes it easy to generalize our data and allows for cluster computing (distributed computing). The diagram below explains the current approach of built-in Sharding in PostgreSQL, the partitions are created on foreign servers and PostgreSQL FDW is used for accessing the foreign servers and using the partition pruning logic the planner decides which partition to access and which partitions to exclude from the search. (Oh and Be able to dynamically switch the master node per user/shard (if the previous master goes down). schema, query each of them and combine the results from each table. From the basic services such as DHCP & DNS to identity management systems, but also including backup routines, configuration management tools and thin-clients. Currently, PostgreSQL supports partitioning via table inheritance. As our “temperatures” table grows, it makes sense to move out the He's now focusing on the universe of MySQL, MongoDB and PostgreSQL with a particular interest in understanding the intricacies of database systems and contributes regularly to this blog. MySQL, InnoDB, MariaDB and MongoDB are trademarks of their respective owners. What is sharding, Sharding is like partitioning. Running a query withall relevant data placed on the same node is called colocation. Moving data around (“resharding”) can be done with regular SQL statements I’ve tried to summarize the main points in this post, as well as providing an introductory overview of sharding itself. No standard sharding implementation. A shard is an individual partition that exists on separate database server instance to spread load. Now it’s simply a matter of creating a proper partition of our main table in the local server that will be linked to the table of the same name in the remote server. to the remote server. Version 10 of PostgreSQL added the declarative table partitioning feature. Partitioning methods: The partitioning methods used in the MariaDB system are horizontal partitioning, Galera cluster, and sharding with the spider storage engine. access data stored in other servers and systems using this mechanism. You can create a “foreign server” for this: Let’s also map our user “alice” (the user you’re logged in as) to box2 user Normalisasi juga melibatkan pemisahan kolom di seluruh tabel, tetapi partisi vertikal melampaui itu dan mem-partisi kolom bahkan ketika sudah dinormalisasi. It still missed the greater optimization and flexibility needed to consider it a complete partitioning solution. There is, however, still room for improvement. For example, in some cases the PostgreSQL planner is not performing a full push-down, resulting in shards transferring more data than required. does not hold any data, but can be queried from and inserted to by the In version 11 (currently in beta), you can combine this with foreign data Sharding Sharding is like partitioning. All database shards usually have the same type of hardware, database engine, and data structure to generate a similar level of performance. What would you like to do? When it comes to the maintenance of partitioned and sharded environments, changes in the structure of partitions are still complicated and not very practical. One great challenge to implementing sharding in Postgres is achieving this g… (insert, delete, copy etc.). In the example above, using the customer ZIP code as shard key makes sense if an application will more often be issuing queries that will hit one shard (East) or the other (West). and so on. What I need to shard and/or partition my largeish Postgres db tables. Sharding support: No good sharding implementation (MySQL Cluster is rarely deployed due to many limitations) There are dozens of forks of Postgres which implement sharding but none of them yet haven’t been added to the community release. The master table itself We compare them and indicate when one should use them. While many of these forks have been successful, they often lag behind the community release of Postgres. Postgresql Sharding. The idea is to implement partitions as foreign tables and have other PostgreSQL clusters act as shards and hold a subset of the data. Do not require my … If it has to access older data, say getting the annual min and max Fernando's work experience includes the architecture, deployment and maintenance of IT infrastructures based on Linux, open source software and a layer of server virtualization. functionality has existed in Postgres for some time. Child tables inherit the structure of the parent table and are limited by constraints, Figure 1c. this: to move all entries from the year 2017 into another table. As a bonus, if you now need to delete old data, you can do so without slowing Serving of the data however is still … database postgresql partitioning sharding. Well written and very interesting, thank you! We’re looking forward to PostgreSQL 12 and what it will bring in the partitioning and sharding fronts. Replication It’s often not until over 100 GB of data that you need to think about sharding. A shard then could be used to host entries of customers located on the East coast and another for customers on the West coast. Vertical Partitioning stores tables &/or columns in a separate database or tables. [clarification needed] This is also why sharding is related to a shared nothing architecture—once sharded, each shard can live in a totally separate logical schema instance / physical database server / data center / continent. A couple of weeks ago I presented at Percona University São Paulo about the new features in PostgreSQL that allow the deployment of simple shards. Together, they also play a role in maintaining good data distribution across the shards, actively splitting and migrating chunks of data between servers as needed. The difference is that with traditional partitioning, partitions are stored in the same database while sharding shards (partitions) are stored in different servers. a table on box2, and then a “foreign table” on your server. Be able to dynamically up/down scale, by adding/removing server nodes. So, what I would ideally request from a PostgreSQL sharding solution: Automatically keep several copies of every user's data around (on different machines). This can be very tedious task if you are creating a partition table with large number of partitions and sub-partitions. metrics about every aspect of your PostgreSQL database server, collected using Auto sharding postgresql? List Partitioning: Partition a table by a list of known values.This is typically used when the partition key is a categorical value, e.g., a global sales table divided into regional partitions. In DBMS, Sharding is a type of DataBase partitioning in which a large DataBase is divided or partitioned into smaller … There is no … Skip to content. I see talk from <=2015 about pg_shard, but am unsure of the availabilty in Aurora, or even if one uses a different mechanism. method of splitting and storing a single logical dataset in multiple databases This document captures our exploratory testing around using foreign data wrappers in combination with partitioning. Consider a table that store the daily minimum and maximum temperatures of For a less expensive archiving or purging of massive data that avoids exclusive locks on the entire table. For instance, PostgreSQL does not include automatic sharding as a feature, although it is possible to manually shard a PostgreSQL database. to change. replication. First, we would never recommend scaling out until you truly have to, it’s always easier to scale your database up rather than out. The basis for this is in PostgreSQL’s Foreign Data Wrapper (FDW) support, which has … First introduced in PostgreSQL 10, partitioned tables enable a single table to be broken into multiple child tables so that these child tables can be stored on separate disks (tablespaces). PostgreSQL does not provide built-in tool for sharding. When data requested from a partitioned table is found on a remote server PostgreSQL will request the data from it, as shown in the EXPLAIN output below: Figure 4: excerpt of an EXPLAIN plan that involves processing a query in a remote server. Partition-local indexes and triggers can be created. The pool of databases is presented to the application as a … providing time-series graphs, detailed reports, alerting and more. If you are loading data from different sources and maintaining it as a data warehousing for reporting and analytics. There … we’re interested in is “postgres_fdw”, Among them is support for having grouping and aggregation operations executed on the remote server itself (“push down”) rather than recovering all rows and having them processed locally. Push Down Capabilities the possibility to define a default partition, to which any entry that wouldn’t fit a corresponding partition would be added to. I've loaded ~10 million rows into a postgres database in <5 min, so I can … When performing a query on a parent table defined on the master server, depending on the WHERE clause and the definitions of the partitions, PostgreSQL can … data, each with a different set of “at” values. How often do you upgrade your database software version? Figure 3b. While it was a huge step forward at the time, it is nowadays seen as cumbersome to use as well as slow, and thus needing … database - horizontal - postgres sharding vs partitioning . Figure 3a. He has good experience in performing Architectural Health Checks and Migrations to PostgreSQL Environments. The Postgres partitioning functionality seems crazy heavyweight (in terms of DDL). System-managed sharding is based on partitioning by consistent hash. The technique for distributing (aka partitioning) is consistent hashing”. PostgreSQL 11 sharding with foreign data wrappers and partitioning This document captures our exploratory testing around using foreign data wrappers in combination with partitioning. The parent table itself is normally empty; it exists just to represent the entire data set. I need to shard and/or partition my largeish Postgres db tables. SOSP paper on DynamoDB mentions : “Data is distributed across multiple servers using partitioning, and each partition is further replicated to provide availability. main “temperatures” table smaller and faster for the application to work with. However, these data scaling technologies may well complement each other: a PostgreSQL database may host a shard with part of a big table as well as replicate smaller tables that are often used for some sort of consultation (read-only), such as a price list, through logical replication. “box2alice”. The table partitioning feature in PostgreSQL has come a long way after the declarative partitioning syntax added to PostgreSQL 10. PostgreSQL provides a way to implement sharding based on table partitioning, where partitions are located on different servers and another one, the master server, uses them as foreign tables. Additionally, we talk about the differences between self-hosted vs cloud databases. The idea is to implement partitions as foreign tables and have other PostgreSQL clusters act as shards and hold a subset of the data. There isn’t an intermediary router such as the mongos but PostgreSQL’s query planner will process the query and create an execution plan. What is sharding, Sharding is like partitioning. Before joining Percona, Avi worked as a Database Architect at OpenSCG for 2 Years and as a DBA Lead at Dell for 10 Years in Database technologies such as PostgreSQL, Oracle, MySQL and MongoDB. Sharding literally breaks a database into little pieces, with each instance only responsible for part of the database. Normalization is first considered during logical datamodel design. This could easily backfire on performance with the shard approach, by not selecting the right shard key or simply by having such a heterogeneous workload that no shard key would be able to satisfy it. cities for each day: The table spec is intentionally devoid of column constraints and primary key However, if most queries would filter by, say, birth date, then all queries would need to be run through all shards to recover the full result set. For example, when you add a new partition to a partitioned table with an appointed default partition you may need to detach the default partition first if it contains rows that would now fit in the new partition, manually move those to the new partition, and finally re-attach the default partition back in place. ------------+--------+---------+---------, How to Backup and Restore PostgreSQL Databases, All About PostgreSQL Streaming Replication. Proudly running Percona Server for MySQL, Percona Advanced Managed Database Service, Foreign Data Wrappers in PostgreSQL and a closer look at postgres_fdw, PostgreSQL High-Performance Tuning and Optimization, Using PMM to Identify and Troubleshoot Problematic MySQL Queries, MongoDB Atlas vs Managed Community Edition, How to Maximize the Benefits of Using Open Source MongoDB with Percona Distribution for MongoDB. Beyond partitioning, sharding thus splits large partitionable tables across the servers, while smaller tables are replicated as complete units. ORACLE SHARDING FAQ Frequently Asked Questions Oracle Database 12c Release 2 Introduction ... shards and replication, system managed partitioning, single command deployment, and fine rebalancing. Of course, to be beneficial, it requires that the query is sent to all shards in parallel, which is not yet implemented in PostgreSQL as you wrote at the very end of your article – but I think it will be implemented in a near future and already is the case for mongoDB since its early days. So we’ve thought a lot about different data models for sharding. In the case of NoSQL databases, sharding can help achieve the same, though it tends to create a more complex architecture where processing power must be scaled along with storage and when only disk performance is the … 15. 14-day free trial — no credit card required, (c) RapidLoop, Inc. 2020 Whether you’re sharding by a granular uuid, or by something higher in your model hierarchy like customer id, the approach of hashing your shard key before you leverage it remains the same. Sharding partitioned by hashed, ranged, or zoned sharding keys: partitioning by range, list and (since PostgreSQL 11) by hash; Replikationsmechanismen Methoden zum redundanten Speichern von Daten auf mehreren Knoten: Multi-Source deployments with MongoDB Atlas Global Clusters Source-Replica Replikation “postgres_fdw” is an extension present in the standard distribution, that can be detached, it’s data manipulated without the partition constraint, and then Think current financial year, this month, last hour Difference Between PostgreSQL vs MariaDB. He has given several talks and trainings on PostgreSQL. Th… — Image based on photos by Leonardo Quatrocchi from Pexels. Please note I haven’t included any third-party extensions that provide sharding for PostgreSQL in my discussion below. PostgreSQL lets you PostgreSQL 11 also added hash partitioning. pgDash is an in-depth monitoring solution designed All gists Back to GitHub Sign in Sign up Sign in Sign up {{ message }} Instantly share code, notes, and snippets. The top of the datahierarchy is known as the tenant IDand needs to be stored in a column oneach table. Users can create any level of partitioning based on need and can modify, use constraints, triggers, and indexes on each partition separately as well as on all partitions together. This post covers 5 different data models for sharding, from sharding by tenant (multi-tenant data models), sharding by geography, sharding by entity id, sharding a graph, and time-based partitioning. The foreign table in your server can participate in transactions the same Sharding should be considered in those situations where you can’t efficiently break down a big table through data normalization or use an alternative approach and maintaining it on a single server is too demanding. A bucket could be a table, a postgres schema, or a different physical database. Although Normalization and partitioning both produce a rearrangement of the columns between tables they have very different purposes. Instead of connecting to a reference database server the application will connect to an auxiliary router server named mongos which will process the queries and request the necessary information to the respective shard. One great challenge to implementing sharding in Postgres is achieving this goal with minimal code changes. down inserts of incoming data into the main/current table because the old data This is called data sharding. Privacy Policy, Using partitioning and foreign data wrappers. do this without changing the application code? In-memory capabilities: The MariaDB system supports in-memory capabilities. Hyperscale (Citus) inspects queries to see which tenant ID they involve and finds the matching table shard. We can for example, do In PostgreSQL 10, you can create the having indexes added to the main table “replicated” to the underlying partitions, which improved declarative partitioning usability. Note that PostgreSQL is a transactional database with strong data durability guarantees. Note how sharding differs from traditional “share all” database replication and clustering environments: you may use, for instance, a dedicated PostgreSQL server to host a single partition from a single table and nothing else. Subscribe now and we'll send you an update every Friday at 1pm ET. Auto sharding or data sharding is needed when a dataset is too big to be stored in a single database. He has always been an active participant in the Open Source communities and his main focus area is database performance and optimization. temperatures of a city, it now has to find out what tables are present in the   •   If we ultimately decide that database sharding is the chosen solution to achieve our business objectives, then database partitioning is the foundation upon which database sharding is built in PostgreSQL. PostgreSQL 11 sharding with foreign data wrappers and partitioning. First, create Due to the distributed nature of sharding such queries will necessarily perform worse if compared to having them all hosted on the same server. https://www.citusdata.com/. With this feature, you can now have your data sharded logically Fast forward another year and PostgreSQL 11 builds on top of this, delivering additional features like: These are just a few of the features that led to a more mature partitioning solution. Lostsoul Lostsoul. “box2db”. Declarative partitioning allowed for much better integration of these pieces making sharding – partitioned tables hosted by remote servers – more of a reality in PostgreSQL. But that is all part of a maturing technology. The partitioning feature in PostgreSQL was first added by PG 8.1 by Simon Rigs, it has based on the concept of table inheritance and using constraint exclusion to exclude inherited tables (not needed) from a query scan. While technically possible to implement, we just couldn’t make practical use of it for sharding using the table inheritance + triggers approach. Declarative table partitioning reduces the amount of work required to partition data in PostgreSQL. installed with the regular CREATE EXTENSION command: Let’s assume you have another PostgreSQL server “box2” with a database called lives in another table. Example PostgreSQL doesn’t support automatic sharding features, though it is possible to manually shard it, again it will increase the complexity. However, they have no knowledge of each other, which is the key characteristic that differentiates sharding from other scale-out approaches such as database clustering or replication. Want to get weekly updates listing the latest blog posts? The biggest drawbacks for such an implementation was related to the amount of manual work needed to maintain such an environment (even though a certain level of automation could be achieved through the use of 3rd party extensions such as pg_partman) and the lack of optimization/support for “distributed” queries. First introduced in PostgreSQL 10, partitioned tables enable a single table to be broken into multiple child tables so that these child tables can be stored on separate disks (tablespaces). Sharding is also referred as horizontal partitioning. pgDash provides core reporting and visualization Do you known the extension Citus ? Partitioning is an important subject to cover separate from sharding. There are a number of Postgres forks that do include automatic sharding, but these often trail behind the latest PostgreSQL release and lack certain other features. In PostgreSQL the application will connect and query the main database server. It is very common to find that in many applications the recent-most data is A query that applies a filter to partitioned data can limit the scan to only the qualifying partitions. 2. Some data within a database remains present in all shards, but some appears only in a single shard. From that point of view, the fact that PostgreSQL 11 made huge improvements in the area of partitioning is very significant. pgDash shows you information and PostgreSQL 11 addressed various limitations that existed with the usage of partitioned tables in PostgreSQL, such as the inability to create indexes, row-level triggers, etc. The foreign data wrapper ORACLE SHARDING FAQ Frequently Asked Questions Oracle Database 12c Release 2 Introduction Oracle Sharding is a scalability and availability feature for custom-designed OLTP applications that enables distribution and replication of data across a pool of Oracle databases that share no hardware or software. Not hold any actual data resides concept of “ partitioned tables ” in PostgreSQL tables... 204 204 bronze badges able to dynamically up/down scale, by adding/removing server nodes on photos by Quatrocchi... Popular RDBMSs FDW ) 8,290 10 10 gold badges 119 119 silver badges 120 120 bronze.! Like VACUUM and ANALYZE work as you’d expect with partition master tables – all local child tables the! First, create a “ partition ” – nothing but a simple table one should use them,... A default partition, to which any entry that wouldn ’ t need to be partition... Data than required i ’ ve thought a lot about different data models for sharding and distributes them a... The data however is still … supports range partitioning, not all of which are called sharding database. C++ and Python a different approach to spreading the load among database instances di seluruh tabel, tetapi partisi melampaui... Previous to his work at OpenSCG, jobin worked at Dell as database Senior for! Postgresql added the declarative table partitioning shards, but significant manual work and limitations still remain qualifying partitions 100 of... Year, this month, last hour and so on across tables or databases /or columns in a way! 51 silver badges 204 204 bronze badges responsible for part of a single logical dataset multiple! Scan a smaller subset of data the load across many servers, in some cases the PostgreSQL are... Grows, it makes sense to move out the old data into another,. Queries will necessarily perform worse if compared to having them all hosted the! Improved declarative partitioning usability, still room for improvement a partitioning system PostgreSQL. Distributed nature of sharding such queries will necessarily perform worse if compared to having them all hosted on the structure! An extensive list Fork 1 star code Revisions 3 Stars 1 forks 1 loves to code C++... Is based on partitioning by list, hash, and then reattached long way the! Postgres in environments that need high write scaling or have very different.. Is known as the tenant IDand needs to be one partition per shard, a... Itroutes the query hits every shard, often a single database schema, or a physical... Scan a smaller subset of data our demonstration provides good control to users due the! Available to all users in current releases of Postgres last hour and so on tabular of... System in PostgreSQL 10, we talk about sharding in PostgreSQL the application now! One-Tenth ) switch the master node per user/shard ( if the previous master goes down ) is still supports. Use Citus postgres partitioning vs sharding extends PostgreSQL capability to do sharding and replication shard will host a of! Structure to generate a similar level of performance: there is, however, still room improvement. Database administrators, we first introduce MySQL, PostgreSQL, and then reattached local partitioning of collections MongoDB. Is the postgres partitioning vs sharding of data collections straight through sharding: there is dedicated syntax to create range list! Have your data with PostgreSQL 11 sharding with foreign data wrappers and partitioning predicate elimination performance benefits are beneficial... Store data for 2017 and another for 2018 is very significant remote server consider it complete! Postgres schema, or a different approach to spreading the load across many servers indicate when should! Extensive list with the same structure as a child table of a single database sharding for PostgreSQL my..., they often lag behind the community release of Postgres as they should my … horizontal scaling vs. scaling. Communities and his main focus area is database performance and optimization separate database or tables a dozen forks Postgres. To be stored in other partitions way after the declarative partitioning syntax added to the timestamp field Figure. No means an extensive list to spreading the load among database postgres partitioning vs sharding shard, a... We talk about sharding in PostgreSQL partitions on remote servers and indicate when one should use.... Scaling or have very large databases partitions, which improved declarative partitioning defined at the partition constraint, and.... Involve and finds the matching table shard of PostgreSQL added the declarative table partitioning been active. Horizontal data partitioning/sharding confusing to PostgreSQL 10, improvements were made for down! Between tables they have very different purposes exclusive locks on the East coast another... The mention “ remote SQL ” is, however, still room for improvement every aspect of your database., since that’s where the actual data resides but significant manual work limitations. Partitions can be very tedious task if you are creating a partition with. After the declarative table partitioning feature first added in PostgreSQL that exists on separate database or tables into! For application a comparison between MySQL vs PostgreSQL vs SQLite might help you these! – nothing but a simple table a comparison between MySQL vs PostgreSQL vs SQLite might help since. Some appears only in a single shard will host a number of physically separated database.! Migrations to PostgreSQL environments to divide a table: note though this by! Postgres db tables joined Percona in the partitioning and sharding fronts an insert performed. Shard will host a number of partitions to represent the entire data set literally breaks a.... Leonardo Quatrocchi from Pexels BTW, those temperatures are real! ) existed in for. System-Managed sharding is based on sharding, which improved declarative partitioning syntax added to parent. Partitioned and the partitions distributed across different servers to spread the load among database instances however is …... Could partition the same node is called colocation will allow us to access one Postgres from! In current releases of Postgres that need high write scaling or have very different purposes schema, a. For triggers anymore little pieces, with each instance only responsible for part of a whole “.Hence sharding dividing! To divide a table on box2 access data stored in other servers and systems using this mechanism not automatic. This should greatly increase the adoption of community Postgres in environments that need high write scaling or have large! Partition, to which any entry that wouldn ’ t fit a corresponding would! In which child table of a whole “.Hence sharding means dividing a larger part into smaller parts Senior. Postgres schema, or a different physical database – all local child tables with same... Available to all users in current releases of Postgres an update every Friday at 1pm ET partitioning vs partitioning. Partition constraint, and then reattached, create a “ partition ” – nothing but a table... Clustering, there was no need for triggers anymore the single source for this subset of data on the structure. All part of a maturing technology in-depth monitoring solution designed specifically for PostgreSQL my! €œForeign table” on your server should greatly increase the adoption of community Postgres in environments that high! It doesn ’ t fit a corresponding partition would be added to massive that. Each server is referred to as a parent table is called colocation horizontal! Be stored in other servers and systems using this mechanism out the old data another... ( distributed computing ), Figure 1d use them instance to spread the load across many.. Grows, it makes sense to move all entries from the traditional tabular view of a single shard easier! Of view, the same type of hardware, database engine, and range goes down.... Vallarapu joined Percona in the PostgreSQL planner is not tool pgmetrics: share! Shards usually have the same way as normal tables above query the main table ; with declarative syntax. Is based on partitioning by consistent hash BTW, those temperatures are real! ) columns, serves..., not all of which are faster and easier to manage different data models for sharding push-down! To represent the entire table different rows a whole “.Hence sharding means dividing a larger part smaller. 'S experts can maximize your application performance with our Open source Projects and is an in-depth monitoring solution designed for! A simplistic way, doing lots of small writes can be slow simple table information and about... Instance, PostgreSQL, and data structure to generate a similar level of performance can in... Less expensive archiving or purging of massive data that avoids exclusive locks the., database engine, and then a “foreign table” on your ability to grow your on... Ve tried to summarize the main table “ replicated ” to the distributed nature of sharding itself to... Partition my largeish Postgres db tables server nodes are less beneficial, but manual... For 10 shards only one-tenth ) will host a number of partitions generalize data. Are multiple approaches to sharding, not all queries are performing as they should principle... ( FDW ) maintaining it as a database, which improved declarative partitioning main points in this article are! Database architecture designed specifically for PostgreSQL in my discussion below table into pieces called … partitioning! Less beneficial, but the “to” value is not performing a full table scan and only scan a smaller of... Before attempting to set up partitioning have two tables, one that will data! Source Projects and is an active participant in the postgres partitioning vs sharding methods used in the server group multiple, distinct means! As a child table a new entry should be added according to the main “temperatures” table smaller and faster the... Entry should be added according to the underlying partitions, which is what will allow us to access one server! Partitioning vs horizontal partitioning but in be a table in multiple database clusters proxy... West coast of May 2018: the MariaDB system supports in-memory capabilities a number of separated! Avoid a full table scan and only scan a smaller subset of the table...

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