Postgresql_ documentation_ 10_ 5.8. schemas bitcoin today rate

Postgresql_ documentation_ 10_ 5.8. schemas bitcoin today rate

5.8.1. Creating a schema

To create a schema, use the CREATE SCHEMA command. Give the

Schema a name of your choice. For example: CREATE SCHEMA myschema;

To create or access objects in a schema, write a qualified name consisting of the schema name

And table name separated by a dot: schema . Table

This works anywhere a table name is expected, including the

Table modification commands and the data access commands

Discussed in the following chapters. (for brevity we will speak

Of tables only, but the same ideas apply to other kinds of

Named objects, such as types and functions.)

Actually, the even more general syntax database . Schema . Table

Can be used too, but at present this is just for

Pro

Forma compliance with the SQL standard. If you

Write a database name, it must be the same as the database you

Are connected to.Bitcoin today rate

So to create a table in the new schema, use: CREATE TABLE myschema.Mytable (

);

To drop a schema if it’s empty (all objects in it have been

Dropped), use: DROP SCHEMA myschema;

To drop a schema including all contained objects, use: DROP SCHEMA myschema CASCADE;

See section 5.13 for a

Description of the general mechanism behind this.

Often you will want to create a schema owned by someone else

(since this is one of the ways to restrict the activities of

Your users to well-defined namespaces). The syntax for that

Is: CREATE SCHEMA schema name AUTHORIZATION user name;

You can even omit the schema name, in which case the schema

Name will be the same as the user name. See section 5.8.6 for how

This can be useful.

Schema names beginning with pg

Are reserved for system purposes and cannot be created by

Users.Bitcoin today rate

5.8.3. The schema search path

Qualified names are tedious to write, and it’s often best

Not to wire a particular schema name into applications anyway.

Therefore tables are often referred to by unqualified names, which consist of just the

Table name. The system determines which table is meant by

Following a search path, which is a

List of schemas to look in. The first matching table in the

Search path is taken to be the one wanted. If there is no match

In the search path, an error is reported, even if matching

Table names exist in other schemas in the database.

The first schema named in the search path is called the

Current schema. Aside from being the first schema searched, it

Is also the schema in which new tables will be created if the

CREATE TABLE command does not

Specify a schema name.

To show the current search path, use the following

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Command: SHOW search path;

In the default setup this returns: search path

————–

$user, public

The first element specifies that a schema with the same name

As the current user is to be searched. If no such schema

Exists, the entry is ignored. The second element refers to the

Public schema that we have seen already.

The first schema in the search path that exists is the

Default location for creating new objects. That is the reason

That by default objects are created in the public schema. When

Objects are referenced in any other context without schema

Qualification (table modification, data modification, or query

Commands) the search path is traversed until a matching object

Is found. Therefore, in the default configuration, any

Unqualified access again can only refer to the public

Schema.

To put our new schema in the path, we use:

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SET search path TO myschema,public;

(we omit the $user here because

We have no immediate need for it.) and then we can access the

Table without schema qualification: DROP TABLE mytable;

Also, since myschema is the

First element in the path, new objects would by default be

Created in it.

We could also have written: SET search path TO myschema;

Then we no longer have access to the public schema without

Explicit qualification. There is nothing special about the

Public schema except that it exists by default. It can be

Dropped, too.

See also section 9.25

For other ways to manipulate the schema search path.

The search path works in the same way for data type names,

Function names, and operator names as it does for table names.

Data type and function names can be qualified in exactly the

Same way as table names. If you need to write a qualified

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Operator name in an expression, there is a special provision:

You must write OPERATOR( schema . Operator )

This is needed to avoid syntactic ambiguity. An example

Is:

SELECT 3 OPERATOR(pg catalog.+) 4;

In practice one usually relies on the search path for

Operators, so as not to have to write anything so ugly as

That.

5.8.5. The system catalog

Schema

In addition to public and

User-created schemas, each database contains a pg catalog schema, which contains the system

Tables and all the built-in data types, functions, and

Operators. Pg catalog is always

Effectively part of the search path. If it is not named

Explicitly in the path then it is implicitly searched

Before searching the

Path’s schemas. This ensures that built-in names will always be

Findable. However, you can explicitly place pg catalog at the end of your search path if

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You prefer to have user-defined names override built-in

Names.

Since system table names begin with pg , it is best to avoid such names to ensure

That you won’t suffer a conflict if some future version defines

A system table named the same as your table. (with the default

Search path, an unqualified reference to your table name would

Then be resolved as the system table instead.) system tables

Will continue to follow the convention of having names

Beginning with pg , so that they

Will not conflict with unqualified user-table names so long as

Users avoid the pg prefix.

If you do not create any schemas then all users access

The public schema implicitly. This simulates the

Situation where schemas are not available at all. This

Setup is mainly recommended when there is only a single

User or a few cooperating users in a database.Bitcoin today rate this setup

Also allows smooth transition from the non-schema-aware

World.

You can create a schema for each user with the same

Name as that user. Recall that the default search path

Starts with $user, which

Resolves to the user name. Therefore, if each user has a

Separate schema, they access their own schemas by

Default.

If you use this setup then you might also want to

Revoke access to the public schema (or drop it

Altogether), so users are truly constrained to their own

Schemas.

To install shared applications (tables to be used by

Everyone, additional functions provided by third parties,

Etc.), put them into separate schemas. Remember to grant

Appropriate privileges to allow the other users to access

Them. Users can then refer to these additional objects by

Qualifying the names with a schema name, or they can put

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The additional schemas into their search path, as they

Choose.

5.8.7. Portability

In the SQL standard, the notion of objects in the same

Schema being owned by different users does not exist. Moreover,

Some implementations do not allow you to create schemas that

Have a different name than their owner. In fact, the concepts

Of schema and user are nearly equivalent in a database system

That implements only the basic schema support specified in the

Standard. Therefore, many users consider qualified names to

Really consist of user name. Table name. This is how

PostgreSQL will effectively

Behave if you create a per-user schema for every user.

Also, there is no concept of a public schema in the SQL standard. For maximum

Conformance to the standard, you should not use (perhaps even

Remove) the public schema.

Of course, some SQL database systems might not implement

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Schemas at all, or provide namespace support by allowing

(possibly limited) cross-database access. If you need to work

With those systems, then maximum portability would be achieved

By not using schemas at all.