RahimullahShaheen commented on issue #354:
URL: https://github.com/apache/age/issues/354#issuecomment-1523413491
**Here is a summary of the first 3 chapters of "The Internals of
PostgreSQL"**
- Chap-1: Database Cluster, Databases, and Table
The first chapter gives us overview about the logical and physical
structures of a postgresql database cluster.
The logical structure of a database cluster consists of databases, which are
collections of database objects which are internally managed by respective
object identifiers (OIDs) and is stored in system catalogs. The physical
structure of database cluster is also described, with each database being a
subdirectory under the base directory and in the subdirectories there are other
files containing particular data and configuration file.
The chapter also gives us information about layout of databases and files
associated with tables and indexes in a postgresql database cluster and about
tablespace. Tablespaces are additional data area outside the base directory to
store data.
The internal layout of Heap Table file is explained later in the chapter,
which is data file divided into pages of fixed length which are numbered
sequentially from 0 and these numbers are called as block numbers.
Finally this chapter explains about the methods of writing and reading
tuples which are record data and are stacked in order from the bottom of the
page within a heap table file.
- Chap-2: Process and Memory Architecture
The second chapter discusses about the process and memory architecture.
A postgresql server contains multiple types of processes to manage database
cluster which are as follow.
The server process which is the parent of all processes is responsible for
starting various background processes and backend processes.
The backend process which is also known as postgres is used for handling
queries issued by connected clients.
The background process is used for performing various task related to
database management like checkpoints, background writer etc.
Finally the chapter explains about the Memory architecture. It is classified
into 2 types, the local memory area is allocated by each backend processes for
querying process and is further divided into several sub-areas whose sizes are
either fixed or variable. The shared memory is allocated by the postgresql
server which is also divided in to several fixed size sub-areas.
- Chap-3: Query Processing
The third chapter explains how queries issued by the connected client are
handled by the backend process.
The backend process consists of five subsystems which are discussed below.
When a query is send to postgres first thing it does is to convert it into a
structure known as parser tree that is more easier for other components to
understand and it is done by the parser.
The analyzer then figures out what the query is trying to do. The optimizer
optimizes the query the best it can and based on the given query postgres might
be capable to execute the query in multiple ways and the planner chooses the
best way for the execution from the available ways. Then finally when the above
steps are done the query is executed.
Further more the chapter discuses the cost estimation of a single query
which includes sort, startup and cost run.
Finally, the 3 join methods supported by the postgresql which are nested
loop, merge and has join, are slighted explained.
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