I have a table that's designed as follows.
master_table
id -> serial
timestamp -> timestamp without time zone
fk_slave_id -> integer
fk_id -> id of the table
fk_table1_id -> foreign key relationship with table1
...
fk_table30_id -> foreign key relationship with table30
Every time a new table is added, this table gets altered to include a new column to link. I've been told it was designed as such to allow for deletes in the tables to cascade in the master.
The issue I'm having is finding a proper solution to linking the master table to the other tables. I can do it programmatically using loops and such, but that would be incredibly inefficient.
Here's the query being used to grab the id of the table the id of the row within that table.
SELECT fk_slave_id, concat(fk_table1_id,...,fk_table30_id) AS id
FROM master_table
ORDER BY id DESC
LIMIT 100;
The results are.
fk_slave_id | id
-------------+-----
30 | 678
25 | 677
29 | 676
1 | 675
15 | 674
9 | 673
The next step is using this data to formulate the table required to get the required data. For example, data is required from table30 with id 678.
This is where I'm stuck. If I use WITH it doesn't seem to accept the output in the FROM clause.
WITH items AS (
SELECT fk_slave_id, concat(fk_table1_id,...,fk_table30_id) AS id
FROM master_table
ORDER BY id DESC
LIMIT 100
)
SELECT data
FROM concat('table', items.fk_slave_id)
WHERE id = items.id;
This produces the following error.
ERROR: missing FROM-clause entry for table "items"
LINE x: FROM string_agg('table', items.fk_slave_id)
plpgsql is an option to use EXECUTE with format, but then I'd have to loop through each result and process it with EXECUTE.
Is there any way to achieve what I'm after using SQL or is it a matter of needing to do it programmatically?
Apologies on the bad title. I can't think of another way to word this question.
edit 1: Replaced rows with items
edit 2: Based on the responses it doesn't seem like this can be accomplished cleanly. I'll be resorting to creating an additional column and using triggers instead.
I don't think you can reference a dynamically named table like that in your FROM clause:
FROM concat('table', rows.fk_slave_id)
Have you tried building/executing that SQL from a stored procedure/function. You can create the SQL you want to execute as a string and then just EXECUTE it.
Take a look at this one:
PostgreSQL - Writing dynamic sql in stored procedure that returns a result set
Related
As school work we're supposed to create a table that logs all operations done by users on another table. To be more clear, say I have table1 and logtable, table1 can contain any info (names, ids, job, etc), logtable contains info on who did what, when on table1. Using a function and a trigger I managed to get the INSERT, DELETE and UPDATE operations to be a logged in logtable, but we're also supposed to keep a log of SELECTs. To be more specific about the SELECTs, in a View if you do a SELECT, this is supposed to be logged into logtable via an INSERT, essentially the logtable is supposed to have a new row with information telling that somebody did a SELECT. My problem is that I can't figure out any way to accomplish this as SELECTs can't make use of triggers and in turn can't make use of functions, and rules don't allow for two different operations to take place. The only thing that came close was using query logs, however as the database is the school's and not mine I can't make any use of them.
Here is a rough example of what I'm working with (in reality tstamp has hours minutes and such):
id operation hid tablename who tstamp val_new val_old
x INSERT x table1 name YYYY-MM-DD newValues previousValues
That works as intended, but what I also need to get to work is this (Note: Whether val_new and old come out as empty or not in this case is not a concern):
id operation hid tablename who tstamp val_new val_old
x SELECT x table1 name YYYY-MM-DD NULL previousValues
Any and all help is appreciated.
Here is an example:
CREATE TABLE public.test (id integer PRIMARY KEY, value integer);
INSERT INTO test VALUES (1,42),(2,13);
CREATE TABLE test_log(id serial primary key, dbuser varchar,datetime timestamp);
-- get_test() inserts username / timestamp into log, then returns all rows
-- of test
CREATE OR REPLACE FUNCTION get_test() RETURNS SETOF test AS '
INSERT INTO test_log (dbuser,datetime)VALUES(current_user,now());
SELECT * FROM test;'
language 'sql';
-- now a view returns the full row set of test by instead calling our function
CREATE VIEW test_v AS SELECT * FROM get_test();
SELECT * FROM test_v;
id | value
----+-------
1 | 42
2 | 13
(2 rows)
SELECT * FROM test_log;
id | dbuser | datetime
----+----------+----------------------------
1 | postgres | 2020-11-30 12:42:00.188341
(1 row)
If your table has many rows and/or the selects are complex, you don't want to use this view for performance reasons.
I have a table contains columns 'employeename' and 'id', how can I sort the 'employeename' column following alphabetical order of the names initial?
Say the table is like this now:
employeename rid eid
Dave 1 1
Ben 4 2
Chloe 6 6
I tried the command ORDER BY, it shows what I want but when I query the data again by SELECT, the showed table data is the same as original, indicting ORDER BY does not modify the data, is this correct?
SELECT *
FROM employee
ORDER BY employeename ASC;
I expect the table data to be modified (sorted by names alphabetical order) like this:
employeename rid eid
Ben 4 2
Chloe 6 6
Dave 1 1
the showed table data is the same as original, indicting ORDER BY does not modify the data, is this correct?
Yes, this is correct. A SELECT statement does not change the data in a table. Only UPDATE, DELETE, INSERT or TRUNCATE statements will change the data.
However, your question shows a misconception on how a relational database works.
Rows in a table (of a relational database) are not sorted in any way. You can picture them as balls in a basket.
If you want to display data in a specific sort order, the only (really: the only) way to do that is to use an ORDER BY in your SELECT statement. There is no alternative to that.
Postgres allows to define a VIEW that includes an ORDER BY which might be an acceptable workaround for you:
CREATE VIEW sorted_employee;
AS
SELECT *
FROM employee
ORDER BY employeename ASC;
Then you can simply use
select *
from sorted_employees;
But be aware of the drawbacks. If you run select * from sorted_employees order by id then the data will be sorted twice. Postgres is not smart enough to remove the (useless) order by from the view's definition.
Some related questions:
Default row order in SELECT query - SQL Server 2008 vs SQL 2012
What is the default SQL result sort order with 'select *'?
Is PostgreSQL order fully guaranteed if sorting on a non-unique attribute?
Why do results from a SQL query not come back in the order I expect?
I am using the COPY command from redshift to copy json data from S3.
The table definition is as follows:
CREATE TABLE my_raw
(
id BIGINT IDENTITY(1,1),
...
...
) diststyle even;
The command for copy i am using is as follows:
COPY my_raw FROM 's3://dev-usage/my/2015-01-22/my-usage-1421928858909-15499f6cc977435b96e610298919db26' credentials 'aws_access_key_id=XXXX;aws_secret_access_key=XXX' json 's3://bemole-usage/schemas/json_schema' ;
I am expecting that any new id inserted will always be > select max(id) from my_raw . In fact it's clearly not the case.
If I issue the above copy command twice, the first time the ids start from 1 to N although that file is creating 114 records(that's a known issue with redshift when it has multiple shards). The second time the ids are also between 1 and N but it took free numbers that were not used in the first copy.
See below for a demo:
usagedb=# COPY my_raw FROM 's3://bemole-usage/my/2015-01-22/my-usage-1421930213881-b8afbe07ab34401592841af5f7ddb31c' credentials 'aws_access_key_id=XXXX;aws_secret_access_key=XXXX' json 's3://bemole-usage/schemas/json_schema' COMPUPDATE OFF;
INFO: Load into table 'my_raw' completed, 114 record(s) loaded successfully.
COPY
usagedb=#
usagedb=# select max(id) from my_raw;
max
------
4556
(1 row)
usagedb=# COPY my_raw FROM 's3://bemole-usage/my/2015-01-22/my-usage-1421930213881-b8afbe07ab34401592841af5f7ddb31c' credentials 'aws_access_key_id=XXXX;aws_secret_access_key=XXXX' json 's3://bemole-usage/schemas/my_json_schema' COMPUPDATE OFF;
INFO: Load into table 'my_raw' completed, 114 record(s) loaded successfully.
COPY
usagedb=# select max(id) from my_raw;
max
------
4556
(1 row)
Thx in advance
The only solution i found to make sure have sequential Ids based on the insertion is to maintain a pair of tables. The first one is the stage table in which the items are inserted by the COPY command. The stage table will actually not have an ID column.
Then I have another table that is the exact replica of the stage table except that it has an additional column for the Ids. Then there is a job that takes care of filling the master table from the stage using the ROW_NUMBER() function.
In practice, this means executing the following statement after each Redshift COPY is performed:
insert into master
(id,result_code,ct_timestamp,...)
select
#{startIncrement}+row_number() over(order by ct_timestamp) as id,
result_code,...
from stage;
Then the Ids are guaranteed to be sequential/consecutives in the master table.
I can't reproduce your problem, however it is interesting how you have identity columns set correctly in conjunction with copy. Here a small summary:
Be aware that you can specify the columns (and their order) for a copy command.
COPY my_table (col1, col2, col3) FROM s3://...
So if:
EXPLICIT_IDS flag is NOT set
no columns listed like shown above
and you csv does not contain data for the IDENTITY column
then the identity values in the table will be set automatically in monotonously as we all want it.
doc:
If an IDENTITY column is included in the column list, then EXPLICIT_IDS must also be specified; if an IDENTITY column is omitted, then EXPLICIT_IDS cannot be specified. If no column list is specified, the command behaves as if a complete, in-order column list was specified, with IDENTITY columns omitted if EXPLICIT_IDS was also not specified.
I'm using Cassandra 1.2.7 with the official Java driver that uses CQL3.
Suppose a table created by
CREATE TABLE foo (
row int,
column int,
txt text,
PRIMARY KEY (row, column)
);
Then I'd like to preform the equivalent of SELECT DISTINCT row FROM foo
As for my understanding it should be possible to execute this query efficiently inside Cassandra's data model(given the way compound primary keys are implemented) as it would just query the 'raw' table.
I searched the CQL documentation but I didn't find any options to do that.
My backup plan is to create a separate table - something like
CREATE TABLE foo_rows (
row int,
PRIMARY KEY (row)
);
But this requires the hassle of keeping the two in sync - writing to foo_rows for any write in foo(also a performance penalty).
So is there any way to query for distinct row(partition) keys?
I'll give you the bad way to do this first. If you insert these rows:
insert into foo (row,column,txt) values (1,1,'First Insert');
insert into foo (row,column,txt) values (1,2,'Second Insert');
insert into foo (row,column,txt) values (2,1,'First Insert');
insert into foo (row,column,txt) values (2,2,'Second Insert');
Doing a
'select row from foo;'
will give you the following:
row
-----
1
1
2
2
Not distinct since it shows all possible combinations of row and column. To query to get one row value, you can add a column value:
select row from foo where column = 1;
But then you will get this warning:
Bad Request: Cannot execute this query as it might involve data filtering and thus may have unpredictable performance. If you want to execute this query despite the performance unpredictability, use ALLOW FILTERING
Ok. Then with this:
select row from foo where column = 1 ALLOW FILTERING;
row
-----
1
2
Great. What I wanted. Let's not ignore that warning though. If you only have a small number of rows, say 10000, then this will work without a huge hit on performance. Now what if I have 1 billion? Depending on the number of nodes and the replication factor, your performance is going to take a serious hit. First, the query has to scan every possible row in the table (read full table scan) and then filter the unique values for the result set. In some cases, this query will just time out. Given that, probably not what you were looking for.
You mentioned that you were worried about a performance hit on inserting into multiple tables. Multiple table inserts are a perfectly valid data modeling technique. Cassandra can do a enormous amount of writes. As for it being a pain to sync, I don't know your exact application, but I can give general tips.
If you need a distinct scan, you need to think partition columns. This is what we call a index or query table. The important thing to consider in any Cassandra data model is the application queries. If I was using IP address as the row, I might create something like this to scan all the IP addresses I have in order.
CREATE TABLE ip_addresses (
first_quad int,
last_quads ascii,
PRIMARY KEY (first_quad, last_quads)
);
Now, to insert some rows in my 192.x.x.x address space:
insert into ip_addresses (first_quad,last_quads) VALUES (192,'000000001');
insert into ip_addresses (first_quad,last_quads) VALUES (192,'000000002');
insert into ip_addresses (first_quad,last_quads) VALUES (192,'000001001');
insert into ip_addresses (first_quad,last_quads) VALUES (192,'000001255');
To get the distinct rows in the 192 space, I do this:
SELECT * FROM ip_addresses WHERE first_quad = 192;
first_quad | last_quads
------------+------------
192 | 000000001
192 | 000000002
192 | 000001001
192 | 000001255
To get every single address, you would just need to iterate over every possible row key from 0-255. In my example, I would expect the application to be asking for specific ranges to keep things performant. Your application may have different needs but hopefully you can see the pattern here.
according to the documentation, from CQL version 3.11, cassandra understands DISTINCT modifier.
So you can now write
SELECT DISTINCT row FROM foo
#edofic
Partition row keys are used as unique index to distinguish different rows in the storage engine so by nature, row keys are always distinct. You don't need to put DISTINCT in the SELECT clause
Example
INSERT INTO foo(row,column,txt) VALUES (1,1,'1-1');
INSERT INTO foo(row,column,txt) VALUES (2,1,'2-1');
INSERT INTO foo(row,column,txt) VALUES (1,2,'1-2');
Then
SELECT row FROM foo
will return 2 values: 1 and 2
Below is how things are persisted in Cassandra
+----------+-------------------+------------------+
| row key | column1/value | column2/value |
+----------+-------------------+------------------+
| 1 | 1/'1' | 2/'2' |
| 2 | 1/'1' | |
+----------+-------------------+------------------+
I have a table in my database and I want for each row in my table to have an unique id and to have the rows named sequently.
For example: I have 10 rows, each has an id - starting from 0, ending at 9. When I remove a row from a table, lets say - row number 5, there occurs a "hole". And afterwards I add more data, but the "hole" is still there.
It is important for me to know exact number of rows and to have at every row data in order to access my table arbitrarily.
There is a way in sqlite to do it? Or do I have to manually manage removing and adding of data?
Thank you in advance,
Ilya.
It may be worth considering whether you really want to do this. Primary keys usually should not change through the lifetime of the row, and you can always find the total number of rows by running:
SELECT COUNT(*) FROM table_name;
That said, the following trigger should "roll down" every ID number whenever a delete creates a hole:
CREATE TRIGGER sequentialize_ids AFTER DELETE ON table_name FOR EACH ROW
BEGIN
UPDATE table_name SET id=id-1 WHERE id > OLD.id;
END;
I tested this on a sample database and it appears to work as advertised. If you have the following table:
id name
1 First
2 Second
3 Third
4 Fourth
And delete where id=2, afterwards the table will be:
id name
1 First
2 Third
3 Fourth
This trigger can take a long time and has very poor scaling properties (it takes longer for each row you delete and each remaining row in the table). On my computer, deleting 15 rows at the beginning of a 1000 row table took 0.26 seconds, but this will certainly be longer on an iPhone.
I strongly suggest that you re-think your design. In my opinion your asking yourself for troubles in the future (e.g. if you create another table and want to have some relations between the tables).
If you want to know the number of rows just use:
SELECT count(*) FROM table_name;
If you want to access rows in the order of id, just define this field using PRIMARY KEY constraint:
CREATE TABLE test (
id INTEGER PRIMARY KEY,
...
);
and get rows using ORDER BY clause with ASC or DESC:
SELECT * FROM table_name ORDER BY id ASC;
Sqlite creates an index for the primary key field, so this query is fast.
I think that you would be interested in reading about LIMIT and OFFSET clauses.
The best source of information is the SQLite documentation.
If you don't want to take Stephen Jennings's very clever but performance-killing approach, just query a little differently. Instead of:
SELECT * FROM mytable WHERE id = ?
Do:
SELECT * FROM mytable ORDER BY id LIMIT 1 OFFSET ?
Note that OFFSET is zero-based, so you may need to subtract 1 from the variable you're indexing in with.
If you want to reclaim deleted row ids the VACUUM command or pragma may be what you seek,
http://www.sqlite.org/faq.html#q12
http://www.sqlite.org/lang_vacuum.html
http://www.sqlite.org/pragma.html#pragma_auto_vacuum