Why does CockroachDB add a rowid column to my tables? They are INT values and do not look ordered, does this column give up the sort order and/or impact how range scans work?
CockroachDB automatically adds a rowid column that serves as a primary key if no primary key is specified for the table. rowid values are generated as a combination of the insert timestamp and the ID of the node executing the statement, as such, ordering is maintained.
To create your own rowid, two functions are commonly used:
unique_rowid(): ensures a unique integer for a primary key, value always increases
unordered_unique_rowid(): ensures a unique integer for a primary key but the rowid value does not always increase. Having rowid values that do not always increase helps divide the key-space more evenly, preventing range hotspots.
Helpful docs from CockroachDB:
Create a table
Auto-generate unique row ids
ID generation functions
Helpful Blog Post:
CockroachDB Key Generation Part 3 - Unordered RowID
Related
I am using auto increment on an integer data column in SQLite. Since it is auto increment, the data is already sorted by that column in ascending order. So, I was wondering if SQLite will perform a binary search over the auto increment column whenever a data is searched by that column.
Effectively yes, but not really.
That is, all AUTOINCREMENT does is add a constraint that requires the value assigned to the column to be higher than any existing value, or higher than any value that has been used, in that column.
But there's more to it than that it is your_column INTEGER PRIMARY KEY (AUTOINCREMENT can only be used on such a column and there can only be 1 such column per table) makes that column an alias of the hidden rowid column.
The rowid is what is indexed, and is basically the most primary index and the most efficient, which always exists unless the table is defined using the WITHOUT ROWID keyword.
So an AUTOINCREMENT column is an alias of the rowid column and uses a differnt, more expensive algorithm than an alias of the rowid without AUTOINCREMENT.
That is without AUTOINCREMENT the value generated for the rowid column will find the maximum value in the table and increment it. Unless that value exceeds 9223372036854775807 in which case SQlite will make some attempts to find an unused lower value (normally between 1 and 9223372036854775807).
With AUTOINCREMENT the algorithm takes the higher value of the maximum value and a value stored in the table sqlite_sequence for the respective table and uses that (thus any deleted higher valueswill not be re-used). However if 9223372036854775807 has been used then an SQLITE_FULL error will be raised.
The following should be noted :-
The AUTOINCREMENT keyword imposes extra CPU, memory, disk space, and
disk I/O overhead and should be avoided if not strictly needed. It is
usually not needed.
SQLite Autoincrement you may well want to read this.
Additional
regrading the comment :-
If I don't use AUTOINCREMENT I have to explicitly create unique
integer IDs and then insert them in database each time a new row is
inserted.
The following demonstrates that there is no requirement for AUTOINCREMENT:-
CREATE TABLE IF NOT EXISTS xyz (ID INTEGER PRIMARY KEY);
INSERT INTO xyz VALUES(null);
SELECT * FROM xyz;
After running twice the result is :-
I've a table TABLE1 with my data. Another clean table TABLE2 as follow:
"TABLE2"(ipt TEXT,instant NUM, id integer auto_increment);
I want to select IP and instant in TABLE1 and insert it into TABLE2 but I don't know why the auto_increment doesn't work.
If someone has an idea.
AUTOINCREMENT can only be used in one situation. and that is for a column defined with a type/constraint of INTEGER PRIMARY KEY.
auto_increment is not a valid keyword and if you used column id integer auto_increment the result would be a column named id with a type of integer auto_increment which would then effectively be a column type of INTEGER i.e. the column's type affinity would be INTEGER. Datatypes In SQLite Version 3
i.e. you must have exactly column_name INTEGER PRIMARY KEY AUTOINCREMENT. (case is irrelevant)
It can also only be coded once per table.
INTEGER PRIMARY KEY with or without AUTOINCREMENT is a special case where the named column is made to be an alias of the rowid column. The rowid being a column that uniquely identifies the row, which is generally hidden (for want of a better description). rowid will not exists if the table is created with the WITHOUT ROWID keyword(s). In which case AUTOINCREMENT cannot be coded within a column definition.Clustered Indexes and the WITHOUT ROWID Optimization
Saying that it is almost certain that you do not in fact need to code AUTOINCREMENT just coding column_name INTEGER PRIMARY KEY will very likely be sufficient and better for your needs. The column will still be given a unique identifier. an integer (64 bit signed) 1 for the first row inserted then likely 2, 3, 4 .....
- Note there is no guarantee that numbering will be sequential/monotonically increased.
Adding AUTOINCREMENT only ensures that the unique identifier is greater and in doing so imposes a limit that the identifier when reaching the highest possible value will subsequently result in an SQLITE_FULL exception, whilst without unused/free values (e.g. deleted rows) that are lower than the highest could be utilised.
To quote the SQLite documentation :-
The AUTOINCREMENT keyword imposes extra CPU, memory, disk space, and
disk I/O overhead and should be avoided if not strictly needed. It is
usually not needed.
SQLite Autoincrement
In teradata why Primary Index must be declared as "non-unique" on all partitioned tables unless the primary index column is also used to define the partition?
I think that the reason is that the insert only goes to the relevant partition. The other partitions don't ever see the row, so they don't have a chance to look for the primary index value and return a uniqueness violation. When the primary index is part of the partition then the uniqueness check can be done because the other partitions won't contain the value of the primary index for the row being inserted. The 1 partition check is all that is needed to guarantee uniqueness.
I have a SQL server table on which I insert account wise data. Same account number should not be repeated on the same day but can be repeated if the date changes.
The customer retrieves the data based on the date and account number.
In short the date + account number is unique and should not be duplicate.
As both are different fields should I concatenate both and create a third field as primary key or there is option of having a primary key on the merge value.
Please guide with the optimum way.
You can create a composite primary key. When you create the table, you can do this sort of thing in SQL Server;
CREATE TABLE TableName (
Field1 varchar(20),
Field2 INT,
PRIMARY KEY (Field1, Field2))
Take a look at this question which helps with each flavour of SQL
How can I define a composite primary key in SQL?
PLEASE HAVE A LOOK, IT WILL CLEAR MOST OF THE DOUBTS !
We can state 2 or more columns combined as a primary key.
In that case every column included in primary key will be called : Composite Key
And mind you Composite keys can never be null !!
Now, first let me show you how to make 2 or more columns as primary key.
create table table_name ( col1 type, col2 type, primary key(col1, col2));
The benefit is :
col1 has value (X) and col2 has value (Y) then no other row can have col1 as (X) and col2 as (Y).
col1, col2 must have some values, they can't be null !!
HOPE THIS HELPS !
Not at all. Just use a primary key constraint:
alter table t add constraint pk_accountnumber_date primary key (accountnumber, date)
You can also include this in the create table statement.
I might suggest, however, that you use an auto-incrementing/identity/serial primary key -- a unique number for each row. Then declare the account number/date combination as a unique key. I prefer such synthetic primary keys for several reasons:
They make it easy to refer to a row in foreign key relationships.
They show the insert order into the table, so you can readily see the last inserted rows.
They make it simple to identify a single row for updates and deletes.
They hide the "id" information of the row from referring tables and applications.
The alternative is to have a PK which is an autoincrementing number and then put a unique unique index on the natural key. In this way uniqueness is preserved but you have the fastest possible joining to any child tables. If the table will not ever have child tables, the composite PK is a good idea. If there will be many child tables, this is could be a better choice.
I have a basic reverse lookup table in which the ids are already sorted in ascending numerical order:
id INT NOT NULL,
value INT NOT NULL
The ids are not unique; each id has from 5 to 25,000 associated values. Each id is independent, i.e., no relationships between the ids.
The table is static. Read only, no inserts or updates ever. The table has 100-200 million records. The database itself will be around 7-12gb. Sqlite.
I will do frequent lookups in this table and want the fastest response time for each query. Lookups are one-direction only, unordered, and always of the form:
SELECT value WHERE id IN (x,y,z)
What advantages does the pre-sorted order give me in terms of database efficiency? What should I do differently than I would with typical unordered tables? How do I tell sql that it's an ordered list?
What about indices: is it necessary or even helpful to create an index on id?
[Updated for clustered comment thanks to Gordon Linoff]. As far as I can tell, sqlite doesn't support clustered indices directly. The wiki says: "Are [clustered indices] supported? No, but if you use INTEGER PRIMARY KEY it acts as a clustered index." In my situation, the column id is not unique...
Assuming that space is not an issue, you should create an index on (id, value). This should be sufficient for your purposes.
However, if the table is static, then I would recommend that you create a clustered index when you create the table. The index would have the same keys, (id, value).
If the table happens to be sorted, the database does not know about this, so you'd still need an index.
It is a better idea to use a WITHOUT ROWID table (what other DBs call a clustered index):
CREATE TABLE MyLittleLookupTable (
id INTEGER,
value INTEGER,
PRIMARY KEY (id, value)
) WITHOUT ROWID;