Here we give a short overview of the stages a query has to pass

<productname>PostgreSQL</productname> is implemented using a

simple <quote>process per user</quote> client/server model. In this model

there is one <firstterm>client process</firstterm> connected to

exactly one <firstterm>server process</firstterm>. As we do not

know ahead of time how many connections will be made, we have to

use a <firstterm>supervisor process</firstterm> (also

<firstterm>master process</firstterm>) that spawns a new

server process every time a connection is requested. This supervisor

process is called <literal>postmaster</literal> and listens at a

specified TCP/IP port for incoming connections. Whenever a request

for a connection is detected the <literal>postmaster</literal>

process spawns a new server process. The server processes

communicate with each other using <firstterm>semaphores</firstterm> and

<firstterm>shared memory</firstterm> to ensure data integrity

throughout concurrent data access.

Once a connection is established the client process can send a query

to the <firstterm>backend</firstterm> (server). The query is transmitted using plain text,

i.e., there is no parsing done in the <firstterm>frontend</firstterm> (client). The

server parses the query, creates an <firstterm>execution plan</firstterm>,

executes the plan and returns the retrieved rows to the client