Does an isolation level still apply until a different one is specified or does it only apply to the next transaction?
For example:
SET TRANSACTION READ WRITE
ISOLTATION LEVEL READ UNCOMMITTED;
SELECT foo FROM aTable;
SELECT bar FROM aTable;
When reading the column bar is the isolation level back to the default serializable?
To make sure I'm not confused, when specifying the isolation level it is dbms wide right?
Related
In my application I am making a query to oracle and getting data this way
<select id="getAll" resultType="com.mappers.MyOracleMapper">
SELECT * FROM "OracleTable"
</select>
I get all the data, the problem is that there is a lot of data and it will take too much time to process all the data at once, since the response from the database will come in 3-4 minutes, this is not convenient.
How to make it so that I receive lines in portions without using the id field (since it does not exist, I do not know why). That is, take the first portion of lines, for example, the first 50, process them and take the next portion. It would be desirable to place a variable in properties that will be responsible for the number of lines in portions.
I can't do this in mybatis. This is new to me. Thanks in advance.
there is such a field and it is unique
OFFSET 10 ROWS
FETCH NEXT 10 ROWS ONLY
don't work, because the version is earlier than 12c
If you want to read millions of rows that's going to take time. It's normal to expect a few minutes to read and receive all the data over the wire.
Now, you have two options:
Use a Cursor
In MyBatis you can read the result of the query using the buffering a cursor gives you. The cursor reads a few hundred rows at a time and your app reads them one by one. Your app doesn't notice that behind the scenes there is buffering. Pretty good. For example, you can do:
Cursor<Client> clients = this.sqlSession.selectCursor("getAll");
for (Client c : clients) {
// process one client
}
Consider that cursors remain open until the end of the transaction. If you close the transaction (or exit the method marked as #Transactional) the cursor won't be usable anymore.
Use Manual Pagination
This solution can work well for the first pages of the result set, but it becomes increasingly inefficient and slooooooow the more you advance in the result set. Use it only as a last resort.
The only case where this strategy can be efficient is when you have the chance of implementing "key set pagination". I assume it's not the case here.
You can modify your query to perform explicit pagination. For example, you can do:
<select id="getPage" resultType="com.mappers.MyOracleMapper">
select * from (
SELECT rownum rnum, x.*
FROM OracleTable
WHERE rownum <= #{endingRow}
ORDER BY id
) x
where rnum >= #{startingRow}
</select>
You'll need to provide the extra parameters startingRow and endingRow.
NOTE: It's imperative you include an ORDER BY clause. Otherwise the pagination logic is meaningless. Choose any ordering you want, preferrably something that is backed up by an existing index.
I create a database and connect with it. But when I execute
select optimizer;
it returns
SELECT: identifier 'optimizer' unknown
What's the problem with it? And I can't find the sys table in the database using \d.
If I want to add an optimizer myopt, is it enough for the steps below:
write the opt_myopt.h and opt_myopt.c in /monetdb5/optimizer/
Add the code into codes in /monetdb5/optimizer/opt_wrapper.c
Add the function into optimizer_init_funcs in /monetdb5/optimizer/optimizer.c
Add a new pipe in /monetdb5/optimizer/opt_pipes.c
Since Oct2020, variables now have a schema (to keep it other SQL objects). In your session, 'sys' is not the session's schema, that's why it cannot find the 'optimizer' variable, the same for the tables.
In default branch (will be available in the next release) I added a "schema path" property on the user to search SQL objects besides the current session's schema. By default it includes the 'sys' schema.
For your first question: if your current_schema is not sys, you need to use select sys.optimizer;.
For your second question: the best existing example is probably in monetdb5/extras/mal_optimizer_template. Next to that, it's basically checking the source code to see how other optimisers have been implemented. NB, although it doesn't often happen, the internals of MonetDB can change between (major) versions. I'd recomment you to use Oct2020 or newer.
Concerning your second question,
You also have to create and add an optimizer pipeline to opt_pipes.c. Look for the default_pipe and then copy/paste that one to a new pipeline and add your optimizer to it.
There are some more places where you might need to add your optimizer, like in the codes[]array in opt_wrapper.c. Just mimick one of the standard optimizers like "reorder".
I have two groups in my crystal report. The First Group is by employee profession and second group is by employee branch. I am able to successfully suppress the groups based on conditions. The problem is if I suppress the branch it works fine as it is child group but when I suppress the Profession it doesn't work properly. It keeps showing me redundant branch as same profession exist in all of the branches. So it is keep repeating the branch for different profession. I actually want to use single group by based on condition on same report.
Is there any way to tackle this problem?
Apply the group suppress logic (in reverse) to the group selection formula.
That would ensure that when a group level 1 is removed, all levels below it are also removed.
Or, if you wish to stay with the section suppress approach, the suppress logic of level 1 needs to be included in the suppress logic of level 2.
Your last sentence is not clear. Perhaps you need to change your grouping logic. Or perhaps you need to use a CrossTab.
I am working with a client new to Power BI, and they complain that when drilling down to the next level, the level above "disappears" showing only the parent of the drill-down records, as shown here:
Top Level Shows All Records
After drill-down, we see the next level down, but only the parent record at the level above:
Here, the lower drill-down records appear, but only their parent appears from the level above.
My client would like the hierarchy to still display the parent levels while expanding only the one child level they are interested in viewing the detail for. I know that entire levels can be expanded at once, but is there a way I can create a matrix that allows the behavior the client is looking for?
Can I drill-down in a Power BI matrix to a lower level while leaving the level above displayed in its entirety?
You should use Expand to the next level, instead of drill down. This should do the work. Try it and let me know if this is the case.
You may find this useful https://learn.microsoft.com/en-us/power-bi/desktop-matrix-visual
A single installation of our product stores it's configuration in a set of database tables.
None of the installations 'know' about any other installation.
It's always been common for customers to install multiple copies of our product in different datacentres, which are geographically far apart. This means that the configuration information needs to be created once, then exported to other systems. Some of the config is then modified to suit local conditions. e.g. changing IP addresses, etc. This is a clunky, error prone approach.
We're now getting requests for the ability to have a more seamless strategy for sharing of global data, but still allowing local modifications.
If it weren't for the local modifications bit then we could use Oracle's data replication features.
Due to HA requirements having all the configuration in the one database isn't an option.
Has anyone else encountered this problem and have you ever figured out a good programmatic solution for this? Know of any good papers that might describe a partial or full solution?
We're *nix based, and use Oracle. Changes should be replicated to all nodes pretty quickly (a second or 2).
I'm not sure how possible it is for you to change the way you handle your configuration, but we implemented something similar to this by using the idea of local overrides. Specifically, you have two configuration tables that are identical (call them CentralConfig and LocalConfig). CentralConfig is maintained at the central location, and is replicated out to your satellite locations, where it is read-only. LocalConfig can be set up at the local site. Your procedure which queries configuration data first looks for the data in the LocalConfig table, and if not found, retrieves it from the CentralConfig table.
For example, if you were trying to do this with the values in the v$parameter table, you could query your configuration using the FIRST_VALUE function in SQL analytics:
SELECT DISTINCT
NAME
, FIRST_VALUE(VALUE) OVER(PARTITION BY NAME
ORDER BY localsort
) VALUE
FROM (SELECT t.*
, 0 localsort
FROM local_parameter t
UNION
SELECT t.*
, 1 localsort
FROM v$parameter t
)
ORDER BY NAME;
The localsort column in the unions is there just to make sure that the local_parameter values take precedence over the v$parameter values.
In our system, it's actually much more sophisticated than this. In addition to the "name" for the parameter you're looking up, we also have a "context" column that describes the context we are looking for. For example, we might have a parameter "timeout" that is set centrally, but even locally, we have multiple components that use this value. They may all be the same, but we may also want to configure them differently. So when the tool looks up the "timeout" value, it also constrains by scope. In the configuration itself, we may use wildcards when we define what we want for scope, such as:
CONTEXT NAME VALUE
------------- ------- -----
Comp Engine A timeout 15
Comp Engine B timeout 10
Comp Engine % timeout 5
% timeout 30
The configuration above says, for all components, use a timeout of 30, but for Comp Engines of any type, use a timeout of 5, however for Comp Engines A & B, use 15 & 10 respectively. The last two configurations may be maintained in CentralConfig, but the other two may be maintained in LocalConfig, and you would resolve the settings this way:
SELECT DISTINCT
NAME
, FIRST_VALUE(VALUE) OVER(PARTITION BY NAME
ORDER BY (TRANSLATE(Context
, '%_'
, CHR(1) || CHR(2)
) DESC
, localsort
) VALUE
FROM (SELECT t.*
, 0 localsort
FROM LocalConfig t
WHERE 'Comp Engine A' LIKE Context
UNION
SELECT t.*
, 1 localsort
FROM CentralConfig t
WHERE 'Comp Engine A' LIKE Context
)
ORDER BY NAME;
It's basically the same query, except that I'm inserting that TRANSLATE expression before my localsort and I'm constraining on Context. What it's doing is converting the % and _ characters to chr(1) & chr(2), which will make them sort after alphanumeric characters in the descending sort. In this way, the explicitly defined "Comp Engine A" will come before "Comp Engine %", which in turn will come before "%". In cases where the contexts are defined identically, local config takes precedence over central ones; if you wanted local to always trump central, even in cases when central was scoped more tightly, you'd just reverse the positions of the two sort terms.
The way we're doing this is similar with Steve's.
First you need a Central Configure Service to save all the configure you want to apply to the distributed environment. Every time you want to modify the config, modify it in the Central Configure Service. In each production host you can write a loop script to update the configure.
For a more sophisticated solution, you need to set up some strategy to avoid a wrong configure batch into all servers, that would be a disaster. Maybe you need a simple lock or a grey release process.