I have a Borland builder c++ 6 application calling Oracle 10g database. Operating over a LAN. When the application in question makes a simple db select e.g.
select table_name from element_tablenames where element_id = 10023842
the following is recorded as happening in Oracle (from the performance logs)
select table_name
from element_tablenames
where element_id = 10023842
then immediately (and not from C++ source code but perhaps deeper)
select table_name, element_tablenames.ROWID
from element_tablenames
where element_id = 10023842
The select statement is only called once in the TADODbQuery object, yet two queries are being performed - one to parse and the other adds the ROWID for executon.
Over a WAN and many, many queries this is obviously a problem to the user.
Does anyone know why this might be happening, can someone suggest a solution?
Agree with Robert.
The ROWID uniquely identifies a row in a table so that the returned record can be applied back to the database with any changes (or as a DELETE).
Is there a way to identify a particular column (or set of columns) as a primary key so that it can be used to identify a row without using a ROWID.
I don't know exactly where the RowID is coming from, it could be either the TAdoQuery implementation or the Oracle Driver. But I am sure I found the reason.
From the Oracle docs:
If the database table does not contain a primary key, the ROWID must be selected explicitly when populating DataTable.
So I suspect your Table does not have a primary key, either add one or add the rowid.
Either way this will solve the duplicate query problem.
Since you are concerned about performance. In general
Using TAdoQuery you can set the CursorType to optimize different behaviors for performance. This article covers this from a TAdoQuery perspective. MSDN also has an article that covers it from from a general ADO Perspective. Finally the specifications from the Oracle Driver can be useful.
I would recommend setting the Cursor to either as they are the only supported by Oracle
ctStatic - Bi-directional query produced.
ctOpenForwardOnly - Unidirectional query produced, fastest but can't call Prior
You can also play with CursorLocation to see how it effects your speed.
Related
I'm using QuestDB and SQL for the first time, and I stumbled upon the LATEST_ON syntax used in QuestDB. Can someone explain it's usage and where to use it?
Quoted from the docs:
For scenarios where multiple time series are stored in the same table, it is relatively difficult to identify the latest items of these time series with standard SQL syntax. QuestDB introduces LATEST ON clause for a SELECT statement to remove boilerplate clutter and splice the table with relative ease.
For more information visit the official documentation
LATEST ON is to find the latest record for each unique time series in a table. See this page for some examples: https://questdb.io/docs/reference/sql/latest-on/
It gives you the latest available record for each combination of the PARTITION BY values, according to the ON timestamp
Maybe easier to understand with an example. If you go to https://demo.questdb.io you can execute this query
select * from trades latest on timestamp
partition by symbol, side
It will then show you the latest existing row for each combination of Symbol and Side. If you wanted to do this using standard SQL, you would probably have to use a window function, something like this
select * from
(select *
,ROW_NUMBER() over (partition by Symbol, Side
order by timestamp DESC) AS RowNumber
from trades where timestamp > '2022-10-01') t
where t.RowNumber = 0
Latest on retrieves the latest entry by timestamp for a given key or combination of keys, for scenarios where multiple time series are stored in the same table.
Check this link for some examples: https://questdb.io/docs/reference/sql/latest-on/
I am currently developing server software in C++ with a MySQL data backend. I am using the official MySQL/connector library from Oracle to work with MySQL. The connection itself is working and I'm not having any issues with that.
My problem is that the database and the table schemas tend to change every once in a while because new tables and columns keep getting added. Also exiting column may be changed for the same reason. To make sure I recognize outdated server software quickly I wanted to add a warning when the database has changed.
My first idea was to hardcode how the database (and tables and such) should look and then check whether the current database matches the hardcoded data. But I have no clue how to achive that.
In summary I want to be able to detect whether
A table has been added or removed
A column in a table has been altered
A column in a table has been added or removed
with as little C++ code as possible. Also it should be quite easy to maintain.
Additional information will be added when required.
I would suggest the following approach:
1) fork and execute the mysql command line client. Set up a pair of pipes, to mysql's standard input and output.
2) At this point you should be able to execute simple commands by piping them to mysql via the standard input pipe, and read the output from the standard output pipe.
You will need to make careful notes as to the output format of each mysql command, so that you know when you finished reading its output, and you can send the next command.
3) As the first order of being, execute:
show tables;
The output that comes back will list all tables in the database. Parsing the output into a list of table names is trival. Then execute for each table:
show create table <tablename>;
The resulting output shows all fields in the table, its keys, and constraints. Pretty much all of this table's schema. Lather, rinse, repeat, for every table.
4) In this manner you can capture a basic schema of the entire database, for comparison purposes. If necessary, use the same approach to capture the triggers, and other objects. You'll likely need to do some minor massaging of the data, and exclude a few bits. "show create table", for example, will include the current AUTO_INCREMENT values, which you can ignore.
This general approach, of driving a mysql process via its standard input and output, is bit wobbly, of course. With a little bit of work, you can use mysql's native client library, and execute all of these commands, and capture their results, directly. This should be more reliable.
Suppose we have a existing siebel column and this column has corresponding mapped eim column also. If I change the length of this siebel base table's column from 100 to 200varhcar by running alter query from backend. How it will impact on the EIM process? Will import process be successful?
Regards,
Robin
If you are interested in knowing conceptually, here are the implications that i can foresee.
a) Table column added using alter table is virtually useless as the application wont be able to use it because its definition is missing from Siebel Repository.
b) If you change the length of an existing column, application would still be using the length mentioned in Siebel Repository.
c) EIM process will ignore your new column length as it loads data dictionary before running the job.
d) And finally, during code migration you have to do the alter table every time since DDLSync process cannot take care of your scenario.
I would advise you not to alter the length of an existing vanilla table column, and instead extend the database table to add a new column. Just as the other poster mentioned, you should do this using Siebel Tools. You will then need to also add reference for this new field into the EIM components (this you also do using Siebel Tools).
This is a best-practice. If your client ever had an Siebel code review done by Oracle, you would be told to do what I described above (not what you were considering doing).
Changing the column length using the alter table command will only change it in the database layer, which will have no repercussions with a siebel standpoint. The EIM tables will still be valid as they will be using the column length mentioned in the repository sent in by tools. If you dont change it in the tools and apply the table, I dont think the changes will work.
I would not recommend that you do this. In this case, probably nothing will go wrong. EIM columns will load data that are upto 100 characters long but from the gui, you could insert upto 200 characters. Something unexpected can go wrong, we would need to know your application better to answer this question.
I was wondering what the "best" way to verify the structure of my database is with SQLite in Qt / C++. I'm using SQLite so there is a file which contains my database, and I want to make sure that, when launching the program, the database is structured the way it should be- i.e., it has X tables each with their own Y columns, appropriately named, etc. Could someone point my in the right direction? Thanks so much!
You can get a list of all the tables in the database with this query:
select tbl_name from sqlite_master;
And then for each table returned, run this query to get column information
pragma table_info(my_table);
For the pragma, each row of the result set will contain: a column index, the column name, the column's type affinity, whether the column may be NULL, and the column's default value.
(I'm assuming here that you know how to run SQL queries against your database in the SQLite C interface.)
If you have QT and thus QtSql at hand, you can also use the QSqlDatabase::tables() (API doc) method to get the tables and QSqlDatabase::record(tablename) to get the field names. It can also give you the primary key(s), but for further details you will have to follow pkh's advice to use the table_info pragma.
I need to make a query like this:
SELECT PNPDeviceID FROM Win32_NetworkAdapter WHERE AdapterTypeId = 0
Trouble is, the AdapterTypeId column isn't always present. In this case, I just want everything, like so:
SELECT PNPDeviceID FROM Win32_NetworkAdapter
My WQL/SQL knowledge is extremely limited. Can anybody tell me how to do this in a single query?
EDIT:
A bit more background seems to be required: I am querying Windows for device information using WMI, which uses an SQL-like syntax. So, in my example, I am querying for network adapters that have an AdapterTypeId of 0.
That column is not always present however, meaning that if I enumerate through the returned values then "AdapterTypeId" is not listed.
EDIT 2:
Changed SQL to WQL; apparantly this is more correct.
I am assuming you mean the underlying schema is unreliable.
This is a highly unconventional situation. I suggest that you resolve the issue that is causing the column to not always be present, because to have the schema changing dynamically underneath your application is potentially (almost certainly) disastrous.
Update:
OK, so WQL lets you query objects with a SQL-like syntax but, unlike SQL, the schema can change underneath your feet. This is a classic example of a leaky abstraction, and I now hate WQL without ever having used it :).
Since the available properties are in flux, I am guessing that WQL provides a way to enumerate the properties for a given adapter. Do this, and choose which query to run depending upon the results.
After some Googling, there is an example here, which shows how to enumerate through the available properties. You can use this to determine if AdapterTypeId exists or not.
SELECT PNPDeviceID FROM Win32_NetworkAdapter WHERE AdapterTypeId = {yourDesire} OR AdapterTypeId IS NULL
I assume that you mean that this field is missing from the table.
Do you know before submitting the query if this field exists?
If yes then just create SQL dynamically, otherwise It think you will get syntax error in case of missing field
This is not an SQL question. SQL does not contemplate records with varying schemas in a single table source. Instead (as you mention) this is a different system using an "SQL-like" syntax. You'll have better luck if you recast the question using the actual product that you're trying to query, and information how that product deals with variable record structures is probably discussed in the documentation.