I know that SQLite does not enforce foreign keys natively, but that's not my primary concern. The question is: If I declare
CREATE TABLE invoice (
invoiceID INTEGER PRIMARY KEY,
clientID INTEGER REFERENCES client(clientID),
...
)
will sqlite at least use the information that clientID is a foreign key to optimize queries and automatically index invoice.clientID, or is this constraint a real no-op?
In the SQLite Documentation it says:
... "an index should be created on the child key columns of each foreign key constraint"
ie. the index is not automatically created, but you should create one in every instance.
Even if it is not actually a no-op (a data structure describing the constraint is added to the table), foreign key related statement doesn't create any index on involved columns.
Indexes are implicitly created only in the case of PRIMARY KEY and UNIQUE statements.
For more details, check it out build.c module on the sqlite source tree:
http://www.sqlite.org/cvstrac/rlog?f=sqlite/src/build.c https://www.sqlite.org/src/file?name=src/build.c&ci=tip
Related
suppose I have a database like:
CREATE TABLE top_table (
test_id PRIMARY KEY,
cmd TEXT);
CREATE TABLE job_table (
id PRIMARY KEY,
rid integer references top_table(rowid));
If I do a vacuum, would it preserve the relationship "rid integer references top_table(rowid)"? Meaning, would it either leave the top_table rowids unchanged, or would it change them and make corresponding changes to rid in job_table? I want to do the vacuum because I'm merging databases, so I read in a second table, insert its data into an existing table, and drop the second table. If the vacuum won't work properly, my next thought was to create the combined database, drop the table, do a sump and read the sql into a new database. Or is there an easier/cleaner method that I'm unaware of?
Using :-
CREATE TABLE top_table ( test_id PRIMARY KEY, cmd TEXT);
CREATE TABLE job_table ( id PRIMARY KEY, rid integer references top_table(rowid));
Could be an issue as you are referencing the rowid directly rather than referencing an alias of the rowid. Typically an alias of the rowid would be used as per :-
CREATE TABLE top_table ( test_id INTEGER PRIMARY KEY, cmd TEXT);
CREATE TABLE job_table ( id INTEGER PRIMARY KEY, rid INTEGER REFERENCES top_table(test_id));
That is specifying INTEGER PRIMARY KEY, instead of PRIMARY KEY (column affinity NUMERIC) is a special case which makes the column being defined an alias of the rowid, the rowid will then not be subject to change by the VACUUM command as per :-
The VACUUM command may change the ROWIDs of entries in any tables that do not have an explicit INTEGER PRIMARY KEY.
SQLITE -CREATE TABLE - ROWID's and the INTEGER PRIMARY KEY
SQLite - VACUUM
I'm somewhat new at this, but strictly speaking, am I using foreign keys? I'm not declaring it to be a foreign key, and "pragma foreign_keys" returns 0
No BUT only because Foreign KEY enforcement is off (hasn't been turned on), thus the REFERENCES top_table(test_id) has no effect, other than being parsed.
Assuming that you programmatically maintain the referential integrity this should not be an issue.
It can be advantageous to have FOREIGN KEY support on as not only will referential integrity be enforced, the ON UPDATE and ON DELETE actions can then be utilised which can simplify handling updates and deletions by using the CASCADE option (e.g. deleting a parent will a) work without a conflict and b) cascade the deletion so that the children rows will be deleted automatically (if they can be))
SQLite - Enabling Foreign Key Support
SQLite - ON DELETE and ON UPDATE Actions
regarding the comment
test_id is a string, which can be relatively long (60-80 characters or so). the original version of the schema had test_id as a member of both tables. the version that I've shown above (with a bunch of fields removed from both tables for this question) was a check on how much smaller the database got if I switched matching long text strings to matching the rowid which seemed to make more sense than adding a field that serves the same purpose)
In the case where a string is used for referencing will not only save space (in the longer term) BUT the overheads will also be greater than using an alias of the rowid.
In the longer term, because SQLite save chunks (pages). A table will (by default) takes up at least 4k and then 8k ....
First, space wise, not only will the extra 52-80 bytes waste space the parent's column should also be unique. If UNIQUE were specified then there is the additional overhead of an index.
However, with an alias of the rowid, then 8 bytes max for the integer, which is used anyway unless the table is defined using WITHOUT ROWID. If I recall there is an extra byte for the flag that the column is an alias of the rowid. The rowid and therefore alias has to be unique and the index (as such) exists. Furthermore due to it being central to SQLite accessing by rowid can be twice as fast.
In short an alias of the rowid is probably the best option in many (probably most) cases for relationships.
In the PostgreSQL 11 Release Notes I found the following improvements to partitioning functionality:
Add support for PRIMARY KEY, FOREIGN KEY, indexes, and triggers on partitioned tables
I need this feature and tested it.
Create table:
CREATE TABLE public.tbl_test
(
uuid character varying(32) NOT null,
registration_date timestamp without time zone NOT NULL
)
PARTITION BY RANGE (registration_date);
Try to create Primary key:
ALTER TABLE public.tbl_test ADD CONSTRAINT pk_test PRIMARY KEY (uuid);
I get an error SQL Error [0A000]. If use composite PK (uuid, registration_date) then it's work. Because PK contains partitioning column
Conclusion: create PK in partitioning tables work with restrictions (PK need contains partitioning column).
Try to create Foreign key
CREATE TABLE public.tbl_test2
(
uuid character varying(32) NOT null,
test_uuid character varying(32) NOT null
);
ALTER TABLE tbl_test2
ADD CONSTRAINT fk_test FOREIGN KEY (test_uuid)
REFERENCES tbl_test (uuid);
I get an error SQL Error [42809]. It means FOREIGN KEY on partitioning tables not work.
Maybe i'm doing something wrong. Maybe somebody tried this functionality and know how this work.
Maybe somebody know workaround except implement constraint in the application.
PostgreSQL v12.0 will probably support foreign keys that reference partitioned tables. But this is still not guaranteed as v12.0 is still in development.
For v11 and lower versions, you may use triggers as described by depesz in these posts: part1, part2, and part3.
Update: PostgreSQL v12.0 was released on Oct 3, 2019, with this feature included
Postgres 11 only supports foreign keys from a partitioned table to a (non-partitioned) table.
Previously not even that was possible, and that's what the release notes are about.
This limitation is documented in the chapter about partitioning in the manual
While primary keys are supported on partitioned tables, foreign keys referencing partitioned tables are not supported. (Foreign key references from a partitioned table to some other table are supported.
(emphasis mine)
I'm new to sqlite and sql in gerneral so I don't know if my approach is reasonable.
I want to model inventory items that can be created, lent, returned and discarded.
I want to model this using two tables, one for items, containing an id, a name and a reference to the last transaction (created, lent, returned, ...) and a table of transactions containing an id transaction type, date, and a reference to the item.
Since creating only one table leaves the database in an inconsitent state with one table referencing a non existant table I thought of using a transaction to crate both tables at once, and defining the foreign keys as deferrable. Creation of a new item would have to be done together in one transaction with creating a "created" event to leave the database in a consistent state.
The following code gives me the error Query Error: not an error Unable to execute multiple statements at a time in sqliteman on linux.
PRAGMA foreign_keys = ON;
begin transaction;
create table items (
id integer primary key,
name char(30),
foreign key (last_transaction) references transactions(transaction_id) DEFERRABLE INITIALLY DEFERRED
);
create table transactions(
transaction_id integer primary key,
text char(100)
foreign key (item) references items(id) DEFERRABLE INITIALLY DEFERRED
);
commit transaction;
Does my approach make sense at all?
If yes, why does the code not work? (Did I make a mistake somewhere, or is what I'm trying impossible in mysql?)
Note: simply creating the tables in one transaction without the foreign key constraints gives the same error. (Could this be a similar Problem to: this question)
I am attempting to use foreign key support in SQLite to maintain referential integrity on a single-table database that has a reflexive join.
e.g.
PRAGMA foreign_keys = ON;
create table tree (
objectId text unique not null,
parentObjectID text,
foreign key (parentObjectID) references tree(parentObjectID) on delete cascade
)
The behavior that I am hoping for is that when a parent row is deleted, its children and their children are deleted as well.
However, when I attempt to delete the root row (where the expected behavior would be that every other row in the database is also deleted), I get this error:
sqlite> delete from tree where objectid = '0';
Error: foreign key mismatch
Are my expectations out of whack with with SQLite foreign key support (and delete behaviors) can provide?
Your problem is pretty simple, your FK on parentObjectId references parentObjectId rather than objectId and SQLite doesn't detect this bit of confusion until you try to use the table. If your FK is defined like this:
foreign key (parentObjectID) references tree(objectID) on delete cascade
From the fine manual:
So, in other words, misconfigured foreign key constraints that require looking at both the child and parent are DML errors. The English language error message for foreign key DML errors is usually "foreign key mismatch" but can also be "no such table" if the parent table does not exist. Foreign key DML errors are may be reported if:
The parent table does not exist, or
The parent key columns named in the foreign key constraint do not exist, or
The parent key columns named in the foreign key constraint are not the primary key of the parent table and are not subject to a unique
constraint using collating sequence specified in the CREATE TABLE, or
The child table references the primary key of the parent without specifying the primary key columns and the number of primary key
columns in the parent do not match the number of child key columns.
The third point would seem to apply here since parentObjectId is neither a PK nor constrained to be unique so that's why you don't see an error until you try to modify the table's content (i.e. use a DML statement rather than a DDL statement).
I have a sqlite table that was originally created with:
PRIMARY KEY (`column`);
I now need to remove that primary key and create a new one. Creating a new one is easy, but removing the original seems to be the hard part. If I do
.indices tablename
I don't get the primary key. Some programs show the primary key as
Indexes: 1
[] PRIMARY
The index name is typically in the [].
Any ideas?
You can't.
PRAGMA INDEX_LIST('MyTable');
will give you a list of indices. This will include the automatically generated index for the primary key which will be called something like 'sqlite_autoindex_MyTable_1'.
But unfortunately you cannot drop this index...
sqlite> drop index sqlite_autoindex_MyTable_1;
SQL error: index associated with UNIQUE or PRIMARY KEY constraint cannot be dropped
All you can do is re-create the table without the primary key.
I the database glossary; a primary-key is a type of index where the index order is typically results in the physical ordering of the raw database records. That said any database engine that allows the primary key to be changed is likely reordering the database... so most do not and the operation is up to the programmer to create a script to rename the table and create a new one. So if you want to change the PK there is no magic SQL.
select * from sqlite_master;
table|x|x|2|CREATE TABLE x (a text, b text, primary key (`a`))
index|sqlite_autoindex_x_1|x|3|
You'll see that the second row returned from my quick hack has the index name in the second column, and the table name in the third. Try seeing if that name is anything useful.