Indexes

Hash Table

PostgreSQL provides the index methods including B-tree, hash, GiST and Gin. User can specify the method through an argument when creating the table. An extra parameter is required for those four methods. User needs to decide how full the index method packs the index page for B-tree, hash and GiST. For heavily updated workload, the parameter should be lower than the read-only workload to reduce the frequency of page split while sacrificing more space. For Gin index, users need to specify whether its fast update option is enabled.

Query Compilation

Not Supported

PostgreSQL currently does not support query compilation. While some researchers do implement LLVM JIT Compilation for PostgreSQL that the expressions in every query are compiled. Their JIT implementation might be included into the master in the future.

Data Model

Object-Relational

PostgreSQL is an object-relational database (i.e., similar to relational database with an object-oriented database model). This means objects are supported in database schemas and query languages. The object-relational database inherits the relational mathematical base with the flexibility to define complex datatypes. Particularly, new types of all objects inside PostgreSQL can be created, including: conversion, cast, functions, data types, domains, procedure languages and indexes.

Stored Procedures

Supported

A stored procedure is defined throught CREATE FUNCTION in PostgreSQL, which allows users to write multiple queries without round trips in a single function. A stored procedure usually returns no value, which means the return value should be specified as NULL. In other cases, which the procedure returns a result set, user should specify the return type as refcursor.

Foreign Keys

Supported

PostgreSQL allows user to define a foreign key constraint, which means the values in that column match the keys in other tables. PostgreSQL also supports a foreign key to contain multiple columns. Besides, a tabel is allowed to define multiple foreign key constraints in PostgreSQL, which is usually used in many-to-many relations among the tables. When the table contains the primary key, which is referenced as foreign key in another table, is deleted, PostgreSQL provides several options, including a) Disallow deleting the referenced table b) Delete the forieng key table as well c) something else.

Views

Virtual Views Materialized Views

PostgreSQL supports both virtual view as well as materialized view. For the virtual one, the create query is run everytime the view is referenced in a trasaction. For materialized view, the query is executed once the command is issued. The materialized view table may be refreshed later by REFRESHING MATERIALIZED VIEW command. PostgreSQL also replaces the view if the same name already existed.

Storage Architecture

Disk-oriented

PostgreSQL stores the table and its index in disk. Memory is used as shared-buffer to accelerate queries. The default size of size of memory is 128 megabytes, which can be changed in runtime config. The user could also changes the memory dedicated for internal operations such as sort, in case the default 4 megabytes are insufficient.

Concurrency Control

Multi-version Concurrency Control (MVCC)

PostgreSQL applies Multi-version Concurrency Control for data consistency. For MVCC, not only the current status but also previous values of data are visible to the transaction, which provides transaction isolations. The primary advantage of MVCC over locking is that the writing operation won't conflict with the reading operation on the same block of data. Thus, MVCC reduces the lock contention to achieve high throughput.

Storage Model

N-ary Storage Model (Row/Record)

PostgreSQL is a row-oriented database server. Every table and its index is stored into a file separately. The file is named by its table or index's filenode number defined in the catalog. Along with the main file, a free space map is also kept to store informations about free space available in the relation. The file is divided to separate segments once its size is larger than 1 GB.

Joins

Nested Loop Join Hash Join Sort-Merge Join

PostgreSQL supports three join implementations: Nested-Loop, Hash and Sort-Merge. When number of join operations is less than a threshold, the default value is 12, PostgreSQL would conduct a near-exaustive to find the cheapest strategy, i.e. all possible plans are genearated to select the best one. When the threshold is exceeded, which suggests searching would be too time-consuming, PostgreSQL would apply heurisitc algorithms to decide join plan.

Logging

Physical Logging

PostgreSQL allows users to control what SQL transactions are logged. The options are none, ddl, mod and all. For example, ddl option would only log the transactions that modify the database, such as CREATE, ALTER and Drop statements. While mod option record all ddl transactions with data-modifying statements such as INSERT, UPDATE and DELETE. The logging takes place when the execute message of a transaction is received.

Checkpoints

Consistent

PostgreSQL supports pg_dump to extract a consistent database into a file while concurrent transactions are running. While pg_dump only outputs a single database, pg_dumpall is used to dump all databases. For checkpoints, PostgreSQL supports periodic checkpointing as well as immediate checkpointing, which both guarantee that previous changes would be written to disk.

Isolation Levels

Read Uncommitted Read Committed Serializable Repeatable Read

User could requst Read Uncommitted, Read Committed, Repeatable Read and Serializable transaction isolation levels in PostgreSQL. However, PostgreSQL only implements three of them. When requesting Read Uncommitted, postgres actually does Read Committed and Dirty Read won't happen. The reason of PostgreSQL to support only three isolation levels is to facilitate the multiversion concurrency control architecture

Query Execution

Tuple-at-a-Time Model

System Architecture

Shared-Disk

PostgreSQL doesn't support multi-master shared-storage but cold standby failure for shared-storage. Which means a secondary server would be the backup of another identical primary system and it's installed and configured once the primary server breaks down. Using PostgreSQL in multi-master shared storage could result in data corruption. Thus, when a primary server is running, the standby server should never access the shared storage.

Query Interface

PL/SQL

PostgreSQL covers a wide range of SQL standards, including operations in transaction-level such as ABORT, BEGIN and END, the Data Definition Languge to create/modify/drop tables, like CREATE, DROP and ALTER as well as Data Manipulation Language, i.e. SELECT, INSERT and DELETE. It also supports procedural languages to create user-defined functions through multiple languages, such as Tcl, Perl and Python.