In Grafana Postgres Datasource has been integrated. For the report Generation, I have added Variables, for example, type. I have used case when clause in this variable filter.
case when type 0 then "ZERO" when type 1 then "ONE", when type 2 then "TWO"
Variable filter works fine in the reporting. In the variable, there is an option to choose All and multi-value.
Again in the Postgres query where clause, I need to convert this to integer,
case when type "ZERO" then 0 when type "ONE" then 1, when type "TWO" then 2.
When I select single value in the filter, ONE or TWO it works fine.
When I select All in the filter, it appends ZERO, ONE, TWO in the value.
The query became
case when type "ZERO, ONE, TWO" then 0
Is there any possible way or remove the option to choose multi value and all from the Grafana variable?
Thank you in advance.
In my tjavaFlex Component Schema, I have set a default value("00") for a column ("RecordCode") in the "Default" column. I'm expecting that this value will take effect when a value is not assigned explicitly to the ("RecordCode") column. But it does not. When does the default value take effect?
Checking the talend documentation
"At present, only tFileInputDelimited, tFileInputExcel, and tFixedFlowInput support default values in the schema."
Not sure they have planned to add support for this in the coming years...
On the one hand the documentation states clearly that
When a column is added with ADD COLUMN and a non-volatile DEFAULT is
specified, the default is evaluated at the time of the statement and
the result stored in the table's metadata. That value will be used for
the column for all existing rows. If no DEFAULT is specified, NULL is
used. In neither case is a rewrite of the table required.
https://www.postgresql.org/docs/12/sql-altertable.html
On the other hand I've heard that because of tuple headers structure in some cases a table might be rewritten.
The documentation states that there are three fields responsible for null values storage behavior
The null bitmap is only present if the HEAP_HASNULL bit is set in
t_infomask. If it is present it begins just after the fixed header and
occupies enough bytes to have one bit per data column (that is, the
number of bits that equals the attribute count in t_infomask2). In
this list of bits, a 1 bit indicates not-null, a 0 bit is a null. When
the bitmap is not present, all columns are assumed not-null.
https://www.postgresql.org/docs/12/storage-page-layout.html
I used this information and pageinspect extension to see what is going on with tuple headers when nullable columns are added or deleted. In all cases when I add or delete nullable column I see that existed tuples never change their headers.
So, I cannot find any evidence to the statement that a table might be rewritten. Fields t_infomask, t_infomask2, t_bits never change their values.
Am I miss something? Can I say that adding nullable columns to high-volume tables is absolutely safe in terms of disk activity (excluding the fact of exclusive locking)?
The behavior has changed in PostgreSQL v11. Before that, adding a column with a non-NULL default will rewrite the table.
The point is that from v11 on, the tuple and its header don't have to be modified, adding such a column is just a metadata change. You won't see that with pageinspect. NULL values are never stored in the tuple, there is just the NULL bitmap, which is present if one of the table columns is nullable.
I am using a tFilterRow to avoid empty rows. While trying to use it I am getting only one function value 'absolute value'.
I want to filter values with a length greater than 0.
Why I am not getting any other functions?
As mentioned in the comments, the length function is only available to schema columns that have the String data type.
To filter out any rows that have a null value in a column you can use a tFilterRow but configured so that the column being checked is not equal to null like so:
In the case you are dealing with the primitive int (rather than the Integer class) then the primitive can never be null and instead defaults to 0 so you'll want to set it as not equal to 0 instead.
Null value means
No value
Inapplicable,unassigned, unknown, or unavailable
Which is true?
It's all about the context in which it's used. A null means there is no value but the reason for this will depend on the domain in which it is being used. In many cases the items you've listed are all valid uses of a null.
It can mean any of those things (and it is not always obvious which), which is one argument against using nulls at all.
See: http://en.wikipedia.org/wiki/Null_(SQL)#Controversy
From Wikipedia
Null is a special marker used in
Structured Query Language (SQL) to
indicate that a data value does not
exist in the database. Introduced by
the creator of the relational database
model, E. F. Codd, SQL Null serves to
fulfill the requirement that all true
relational database management systems
(RDBMS) support a representation of
"missing information and inapplicable
information". Codd also introduced the
use of the lowercase Greek omega (ω)
symbol to represent Null in database
theory. NULL is also an SQL reserved
keyword used to identify the Null
special marker.
Obviously you have the DB definition of what null means, however to an application it can mean anything. I once worked on a strange application (disclaimer- I didn't design it), that used null in a junction table to represent all of the options (allegedly it was designed this way to "save space"). This was a DB design for user and role management.
So null in this case meant the user was in all roles. That's one for daily WTF. :-)
Like many people I tend to avoid using nulls where realistically possible.
null indicates that a data value does not exist in the database, thus representing missing information.
Also allows for three-way truth value; true, false and unknown.
The only answer supported by SQL semantics is "unknown." If it meant "no value," then
'Hi there' = NULL
would return FALSE, but it returns NULL. This is because the NULL value in the expression means an unknown value, and the unknown value could very well be 'Hi there' as far as the system knows.
NULL is a representation that a field has not had a value set, or has been re-set to NULL.
It is not unknown or unavailable.
Note, that when looking for NULL values, do not use '=' in a where clause, use 'is', e.g.:
select * from User where username is NULL;
Not:
select * from User where username = NULL;
NULL, in the relational model, means Unknown. It's a mark that appears instead of a value wherever a value can appear in SQL.
Null means nothing, unknown and no value.
It does not mean unavailable or in applicable.
Null is a testable state of a column in a row, but it has no value itself.
By example:
An int can be only ...,0,1,2,3,... and also NULL.
An datetime can be only a valid date... and also NULL.
A bit can be only 0 or 1... and also NULL.
An varchar can be a string... and also NULL.
see the pattern?
You can make a column NOT NULL-able so that you can force a column to take a value.
The NULL SQL keyword is used to represent either a missing value or a value that is not applicable in a relational table
all :-)
if you want to add a semantic meaning to your field, add an ENUM
create TABLE myTable
(
myfield varchar(50)
myfieldType enum ('OK','NoValue','InApplicable','Unassigned','Unknown','Unavailable') NOT NULL
)