As part of the migration from Oracle ACS1 to SQL Server, it was necessary to build a tool that could quickly copy the content of ACS1 tables to converted versions of the same table on SQL Server. The Oracle and SQL Server tables have the same names, column names, and the same or at least a compatible type. There are ways to copy Oracle to Oracle and SQL Server to SQL Server, but you have to write a program to copy tables across different databases. While Talend does data copying in production, a developer needs something they can run on demand. The same program can be run in a way to compare the two tables and report differences.
While ACS1 is in production, the tool is used to copy the real data to a development SQL Server. As we migrate some functions to SQL Server, specific groups of tables will be copied back from the now authoritative SQL Server to Oracle to support anyone with an old Oracle based reporting tool. Eventually the Oracle database will go away.
Because Yale applications are written in Java, it is natural to consider writing the function in that language. Although JDBC is portable and supports both types of databases, Java is still a strongly typed language which may add a third point of conversion (Oracle to Java to SQL Server rather than just Oracle to SQL Server).
Powershell is an interpreted language built on top of the .NET framework. However, in normal processing Powershell sets up but does not even participate in the data copy. It provides each database with a connection string and authenticates with a userid having appropriate privileges to read and write data.Script in Git: https://git.yale.edu/gilbert/DBScripts/blob/main/CopyTableRows.ps1
Microsoft has a bcp.exe program to copy a table from one SQL Server database to another, and if you link one database server to another you can copy both the definition and the contents of a table from one server to another using “SELECT INTO” or copy data from one table to another using “INSERT SELECT”.
However, things become more complicated if you are moving tables or data from one type of database to another. The requirement to migrate the Oracle ACS1 database to SQL Server produced a Powershell script that can be more generally used to synchronize information between databases and support Sandbox development and reporting.
Since our applications are written in Java, not use it? Powershell is based on the .NET framework, and while both Java and .NET have a similar plug in design for database drivers, .NET has full table operations that simply copying and migration.
The SQL for CREATE TABLE is much the same in all databases. The exact Types of columns may need adjustment because specific Type names mean different things in different systems. That is a manual process that you have to work out on your own. We assume you have generated an equivalent definition of each table in a target output system. The columns have the same name, and the types are similar enough that data can be automatically converted from one to the other. NUMERIC, DECIMAL, TINYINT, SMALLINT, INT, or BIGINT are different sizes of numbers, CHAR, VARCHAR, VARCHAR2, NVARCHAR are different character strings with different sizes and maybe character sets. Dates and timestamps will similarly convert even though different systems may use different starting dates and precisions.
There is no automatic way to handle data types like Blobs, Clobs, XML, etc. If you have columns of these types, you may need to write special conversion rules anyway.
While we want to copy the exact data between production systems, when we are moving data to a TEST or developer Sandbox, we may want to audit or transform the contents. For testing, we may want to assign a single common dummy SSN or Date of Birth to everyone, since the actual values are not needed for testing and we then reduce the sensitivity of the copied tables.
During initial development the script can be used to periodically refresh the important tables from a source database to another. However, as we start to migrate application, different components will move at different times. The tool can synchronize some Oracle tables to SQL Server (to test new code) and some SQL Server tables back to Oracle (to support unconverted legacy applications and reports after the production application has moved to SQL Server).
The script can also be used for ad hoc reporting. For example, an analyst can capture point int time snapshots of data, compare the before and after, and generate reports on the changes over time.
Read and Write entire Tables
.NET defines some abstract classes of objects that perform generic functions. Oracle and SQL Server provide drivers that implement these template classes. When the script configures Oracle as the input connection and asks for a DataAdapter object, Oracle complied code provides it. Similarly, the SQL Server output connection provides a SQL Server BulkCopy object.
The only parameter to the DataAdapter is a SELECT statement to read data from a table. In most cases it is just “SELECT * FROM tablename”. The DataAdapter communicates with the database, reads metadata about the named table, and discovers the column names and types for you.
The script has to create one generic .NET object, and empty DataSet object. This is an empty collection that can hold a .NET DataTable object.
The script can then pass the empty .NET DataSet to the Oracle DataAdapter.Fill(DataSet) method. Oracle then reads the entire table into memory, creating one DataTable object with one DataRow object for each row in the table.
The table was read by Oracle and stored in objects of whatever type Oracle chose based on the definition of the table and its columns in its database. The Powershell script does not need to know how many columns or rows there are, nor the names or type of anything. The most it has to do is select the first DataTable in the DataSet collection and then pass it to the BulkCopy.WriteToServer(DataTable) method.
With this call, the SQL Server BulkCopy object looks at the DataTable and DataRows that Oracle created. It also can read the metadata from the SQL Server database to discover how the output table is defined. It develops a strategy for using the objects in memory to populate the output table.
As long as the Oracle and SQL Server tables have the same number of columns, with the same names and compatible types, the data can be copied as is or with whatever transformations the database driver regards as necessary. For example, an Oracle NUMERIC or DECIMAL column can be converted to a SQL Server INT or BIGINT column, but that is up to the databases and does not involve the script programming.
All the real work is done by compiled code provided by the database vendors. Essentially the script simply introduces Oracle to SQL Server, tells them to copy the data, and then they make all the decisions until the operation has completed.
BulkCopy.WriteToServer by default temporarily disables triggers and optimizes the evaluation of constrains. As the name suggests, it moves the data in bulk rather than doing one INSERT per row.
BulkCopy can append data to a table that is already filled, but when copying an entire table, the script will typically TRUNCATE the output table before refilling it with BulkCopy. If other programs may be accessing the database at the same time, the entire operation can be performed in a Transaction.
In the normal case, the script is trivial. Additional code has been added, but it is optional and used only to meet special needs. For example, the script can be run to read both the input and output tables into memory, perform a generic object compare by primary key of the corresponding rows in each table, and then either report the differences or make only the individual changes needed to synchronize the output with the input. For this to work, the data types of the two tables have to be identical so the values of the data in each row can be compared at the binary level.
Alternately, the script can be run to compare the data in each cell of each table. Running nested loops in an interpreted language like Powershell uses a lot of time, but this is just CPU on the desktop computer which costs nothing and can be run in the background. The advantage here is that Powershell as an interpreted language with weak, dynamic typing will convert compatible types as needed to determine if the values are equivalent even if they are not identical at the binary level.
The simplest and fastest thing is to TRUNCATE the output and copy all the data.
If you also want a report, you can compare and only update changed rows, and then the script will tell you the number of rows modified, added, or deleted since the last copy. If you just want the report, turn on the Read-Only flag and it will not change the “output” table.
The script can be modified to add custom conversions if the default behavior is inadequate. However, before writing a lot of code, you can write a custom SELECT that creates the input DataAdater, and in the SQL you can CAST or CONVERT the data in certain columns to a more appropriate type. This is simpler and runs much faster than Powershell loops.
Two .NET templates that Java JDBC does not have are the DataAdapter and BulkCopy.
You provide a text string containing a SELECT statement to a DataAdapter. It discovers the names and types of the columns from metadata supplied by the database, then reads the entire result into objects in memory. The .NET program can then work on the entire result as a Collection of objects.
The Row objects act like “Entities”. If you change data in a Row, or delete the Row, or add a new Row, the objects in memory track the changes you made. When you are done, you can tell the DataAdapter to alter the database table to match the changes you just made to the memory object, and the DataAdapter generates whatever INSERT, UPDATE, and DELETE operations are needed. (Of course, for this to work you either have to lock the result or be the only program making changes to it during the period).
Alternately, BulkCopy inserts new rows into a table, but it is optimized to transfer all the rows in a single operation rather than doing a sequence of INSERT statements each of which has to be sent and then the return has to be checked. BulkCopy can fill an empty table or append new rows after existing data. Triggers and Constraints are also disabled or optimized as you choose.
Since the objects in memory are the same no matter what database system you are using, .NET defines templates for a DataSet, which is a generic collection of DataTables, which are generic collections of DataRows which are a collection of Items.
Normally, however, the script does not have to not deal with anything below the level of the entire table.
An example of the normal use of the script to copy all data in an Oracle table to a SQL Server table (which may have an older copy of the data in it) is to
Connect to the two databases with userids and password. The Oracle userid has to be able to read the table and the SQL Server userid has to be able to write to the table.
Have Oracle create a DataAdapter for the input table based on a “SELECT *”. This discovers the definition of the input table in the Oracle database including its column names and types.
Have SQL Server create a BulkCopy for the output table.
Create an empty DataSet object in memory.
Call the DataAdapter.fill() method passing the empty DataSet. This reads the table into memory generating one DataTable with DataRows containing the contents of the table. Oracle decides what native binary data types to use to hold each of the values in each row.
Start a Transaction on the output database.
TRUNCATE the output table to delete all the old data.
Call BulkCopy.WriteToServer() passing the DataTable that Oracle generated. At this point the classes that implement the SQL Server BulkCopy object match the DataTable and DataRows created by Oracle to the column types in SQL Server and do any necessary type conversion, then send all the data out to the database to repopulate the table.
Commit the Transaction to swap the old and new version of the output table.
You can now close the connections or copy another table.
There are alternate Copy-DBTable functions or options of a function that combine steps 2 through 9. Any table can be emptied and repopulated.
If the tables have a unique key, then there is an option to read both the input and output tables into memory, compare the rows to see which ones are different, and then use an output DataAdapter instead of a BulkCopy to only generate INSERT, UPDATE, or DELETE for the changed rows. This code had to be written to see if it was better, but it turned out that TRUNCATE and rewrite is just as fast and a lot simpler.
If the default version of the data provided by the input database doesn’t work, the best option is to write a custom SELECT statement that transforms columns as they are read in. This avoids writing loops in Powershell. However, there are examples of using Powershell to examine values of selected columns of every row.
The normal case is just to let the database drivers do the work, and since they are the same drivers and this is the same .NET framework that every application would have to use, there is no performance loss using interpreted Powershell instead of a compiled language.
Use
Download the script from Git.
The input and output databases must have the same table name, with the same column names and compatible types.
Create a connection to the input and output databases. Then call the functions in the script to copy tables.
The following code truncates (empties) EXAMPLE.TABLE in the output database, then replaces its contents with the rows of the same table in the input database.
| Code Block |
|---|
. .\CopyTableRows.ps1
# Connect to an Oracle input database. For protection we separate the Userid (who should have read-only access) from the password
$inConnectionString = 'User Id={0};Password={1};Data Source=server.its.yale.edu/xxx' -f 'inuser', 'inpassword'
$inconn = New-Object Oracle.ManagedDataAccess.Client.OracleConnection($inConnectionString)
$inconn.Open() # Get a backing connection from the Oracle pool manager to reactivate the OracleConnection object
# Connect to a SqlSrv output database using the current Windows login as the credential
$outconn = New-Object System.Data.SqlClient.SqlConnection('Server=localhost;Integrated Security=true;Database=xxx;')
$outconn.Open()
Copy-DbTable -InputTableName 'EXAMPLE.TABLE' -InConn $inconn -OutConn $outconn -truncate
# Repeat as needed
$inconn.Close()
$outconn.Close() |
Considerations
Performance
Suppose you are copying a table every night and typically it only has a few dozen rows changed since the last copy. Is it better to compare the two tables and only make the necessary changes?
It was necessary to write the script and try both a full copy and an incremental update. It turns out that there is very little difference between the time it takes to perform each approach. It takes the same amount of time to read a whole table and write a whole table as it does to read the table twice from two sources.
Therefore, the choice may depend on special factors about the table. Are there constraints, indexes, triggers, or something else that makes it better to make the smallest number of changes possible? If not, then the full copy works as well as anything else.
Primary Key
In order to compare the input and output rows, the input and output tables must have Primary Keys. If you use the -truncate option and empty the output table and replace all the rows, then there is no key requirement.
Copy-History
Important “base” tables may have an associated History table. When any row is changed or deleted, the previous data in the row is copied to a new row at the end of the corresponding History table. This can happen automatically if there is a Trigger on the base table that is called with each UPDATE or DELETE.
The best practice is for rows in the History table to have an “identity” column, where a new sequential integer is generated each time a row is added. However, we have found that existing Oracle applications may not have any unique row identifier. It appears that in both Oracle and SQL Server new rows are appended to “the end of the table” and when reading rows, they are by default returned chronologically. This is regarded as bad practice in SQL textbooks.
If you use -truncate then you can treat history tables like any other table.
The Copy-History function provides experimental support for identifying the new records added onto the end of an Oracle history table and appending copies of just those records onto the end of a SQL Server history table. The copy works only in the one direction, because only Oracle has a SQL operation to skip over the old rows.
To copy a History table from SQL Server to Oracle, you must specify -truncate and copy the whole table from the beginning.
Earliest DATE
While both Oracle and SQL Server support DATE values going back millennia, the .NET SqlDateTime class has a minimum date of 1/1/1753. This tripped us up because a few Yale tables set unknown dates to the day that Yale was founded, Oct 5, 1701. That value could not be converted by Microsoft utilities and cannot be copied by this script. To get around this, a custom SELECT statement is created in the calling program and is passed with the optional parameter -SelectSQL. This SELECT statement contains expressions of the form:
GREATEST(CHANGE_EMAIL_ADDRESS_DATE,DATE '1776-07-04') AS CHANGE_EMAIL_ADDRESS_DATE
So that the earliest date allowed is July 4, 1776, but you could choose any other date after 1753 if you prefer.