Recently Which datatype is used for generating sequence in postgres13 ?
Creating Sequence separately or Using serial or Identity datatype ?
Which is best and why ?
Also Mapping between datatype is easy ?
As documented in the manual default data type is bigint if you don't specify a type.
The optional clause AS data_type specifies the data type of the sequence. [...] bigint is the default. The data type determines the default minimum and maximum values of the sequence.
(emphasis mine)
However with a current Postgres version it is recommended to use identity columns instead of serial
As documented in the manual you can control the details (data type most importantly) of the underlying sequence when you declare an identity column.
Related
CREATE temp TABLE demo1(
col_a int,
col_b int DEFAULT (col_a)
);
PostgreSQL have generated columns, triggers. But why cannot use column reference in DEFAULT expression.
since the manual ddl-default page mention:
The default value can be an expression, which will be evaluated
whenever the default value is inserted (not when the table is
created). A common example is for a timestamp column to have a default
of CURRENT_TIMESTAMP, so that it gets set to the time of row
insertion.
Obviously, column references is value expression.
https://www.postgresql.org/docs/current/sql-expressions.html#SQL-EXPRESSIONS-COLUMN-REFS
A value expression is one of the following:
* A constant or literal value
The documentation states:
DEFAULT default_expr
The DEFAULT clause assigns a default data value for the column whose column definition it appears within. The value is any variable-free expression (in particular, cross-references to other columns in the current table are not allowed). Subqueries are not allowed either. The data type of the default expression must match the data type of the column.
So the restriction is clearly documented.
But it is fairly easy to see that it would pose problems to allow a column reference in a DEFAULT expression: For one, the value specified in the INSERT statement could be overridden by BEFORE INSERT triggers. In that case, should the value before or after the trigger execution be used? Also, what if the DEFAULT for column a references column b and vice versa? There may be other difficulties that I didn't think of.
Currently I'm using serial datatype for my project. I want to know the following details to change previously Created SEQUENCE datatype to SERAIL datatype.
From Which version onwards POSTGRESQL supports SERAIL DATATYPE, also If I change my id into SERAIL it wont affect my future code.
What is the max size of serial and its impact?
Can ALTER SEQUENCE impact the Sequence numbers?
any drawback on serail datatype in future?
How to create gap-less sequence?
All answers can be found in the manual
serial goes back to Postgres 7.2
It's a bigint, the max size is documented in the manual. Also see this note in the documenation of CREATE SEQUENCE
Sequences are based on bigint arithmetic, so the range cannot exceed the range of an eight-byte integer (-9223372036854775808 to 9223372036854775807).
Obviously. As documented in the manual that command can set minvalue or restart with a new value and manipulate many other properties that affect the number generation.
You should use identity columns instead
Not possible - that's not what sequences are intended for. See e.g. here for a possible implementation of a gapless number generator.
Using EF-Core for PostgresSQL, I have an entity with a field of type byte but decided to change it to type byte[]. But when I do migrations, on applying the migration file generated, it threw the following exception:
Npgsql.PostgresException (0x80004005): 42804: column "Logo" cannot be
cast automatically to type bytea
I have searched the internet for a solution but all I saw were similar problems with other datatypes and not byte array. Please help.
The error says exactly what is happening... In some cases PostgreSQL allows for column type changes (e.g. int -> bigint), but in many cases where such a change is non-trivial or potentially destructive, it refuses to do so automatically. In this specific case, this happens because Npgsql maps your CLR byte field as PostgreSQL smallint (a 2-byte field), since PostgreSQL lacks a 1-byte data field. So PostgreSQL refuses to cast from smallint to bytea, which makes sense.
However, you can still do a migration by writing the data conversion yourself, from smallint to bytea. To do so, edit the generated migration, find the ALTER COLUMN ... ALTER TYPE statement and add a USING clause. As the PostgreSQL docs say, this allows you to provide the new value for the column based on the existing column (or even other columns). Specifically for converting an int (or smallint) to a bytea, use the following:
ALTER TABLE tab ALTER COLUMN col TYPE BYTEA USING set_bytea(E'0', 0, col);
If your existing column happens to contain more than a single byte (should not be an issue for you), it should get truncated. Obviously test the data coming out of this carefully.
Currently creating my PostgreSQL tables through Postico, and I came across this field for when creating new columns. It is called DEFAULT and its default value is no default. You can select constant, expression, and sequence as options, though.
What exactly do these mean?
The manual on CREATE TABLE:
DEFAULT default_expr
The DEFAULT clause assigns a default data value for the column whose column definition it appears within. The value is any
variable-free expression (subqueries and cross-references to other
columns in the current table are not allowed). The data type of the
default expression must match the data type of the column.
The default expression will be used in any insert operation that does
not specify a value for the column. If there is no default for a
column, then the default is null.
constant and expression should be clear now. sequence is a special feature to make it a serial column:
Creating a PostgreSQL sequence to a field (which is not the ID of the record)
More details on the page #mu provided:
https://www.postgresql.org/docs/current/static/ddl.html
How I can set the default value for an attribute to be the next value of a squeuence
I tried the following :
ALTER TABLE Person ADD COLUMN ccn VARCHAR(254) default (next value for KAP.CCN_SEQ );
And this cause a syntax error
Could you please advice is this doable and if yes what is the correct syntax ?
Thanks in advance.
Your column is of type Varchar - I assume this is a bit unusual for a sequence number. You also need to determine whether you want to create an Identity or a Sequence. For difference between the two and how to create each see: ID vs Sequence in DB2. Some of the points to note are (as quoted from the ref.):
An identity column has the following characteristics:
• An identity column can be defined as part of a table only when the table is created. Once a table is created, you cannot alter it to add an identity column. (However, existing identity column characteristics may be altered.)
• An identity column automatically generates values for a single table.
• When an identity column is defined as GENERATED ALWAYS, the values used are always generated by the database manager. Applications are not allowed to provide their own values during the modification of the contents of the table.
A sequence object has the following characteristics:
• A sequence object is a database object that is not tied to any one table.
• A sequence object generates sequential values that can be used in any SQL statement.
• Since a sequence object can be used by any application, there are two expressions used to control the retrieval of the next value in the specified sequence and the value generated previous to the statement being executed. The PREVVAL expression returns the most recently generated value for the specified sequence for a previous statement within the current session. The NEXTVAL expression returns the next value for the specified sequence. The use of these expressions allows the same value to be used across several SQL statements within several tables.
To create a sequence (example):
CREATE SEQUENCE ccn
START WITH 1
INCREMENT BY 1
NOMAXVALUE
NOCYCLE
CACHE 24