About subsetting

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What is subsetting?

Subsetting allows you to intelligently reduce the size of your destination database. It takes a representative sample of the data while preserving the data's referential integrity.

You configure how Tonic Structural should generate a subset. When you generate the output data, you decide whether to enable the subsetting process.

For example, you can configure subsetting to get 5% of all transactions, or all of the data that is associated with customers who live in California.

Here are a few examples where subsetting data might be important or necessary:

  • You want to use your production database in staging or test environments, without the PII. The database is very large, so you want to use only a portion of it.

  • You want a test database that contains a few specific rows from production (and related rows from other tables) so that you can reproduce a bug.

  • You want to share data with others but you don’t want them to have all of it. For example, you can provide developers an anonymized subset that also enables them to run the test database locally on their local machines.

To learn more about our approach to subsetting, go to the following technical blog posts:

Components of subsetting

Subsetting uses foreign keys to determine the relationships in the data. These relationships enable the subsetting process to traverse the database as it builds the subset. Foreign keys are either configured in your source data, or configured using the Structural virtual foreign key tool. For more information, go to Subsetting and foreign keys.

For subsetting, each table in the source database falls into one of the following categories:

Target tables

Target tables are the seed tables that provide the initial set of rows to include in the subset. Structural retrieves the initial subset of data from the target tables. Structural then uses those rows to identify the information to pull from related tables.

A target table typically contains an important object that is well connected to everything else in the source data. For example, users, transactions, or claims. A subset should usually have a very small number of target tables.

When you identify a target table, you specify how to retrieve the subset of the data that you want from the table. You can request a percentage of the data, or use a WHERE clause to identify a specific subset of data.

For more information, go to Identifying and configuring target tables.

Lookup tables

A lookup table contains a static set of values that is used in other tables in your subset. For example, a lookup table might contain a list of postal codes or country names that are referenced in other tables.

Structural always retrieves all of the data in a lookup table. It does not check whether or where the lookup values are used.

It does not pull records from related tables based on lookup table values. Relationships with lookup tables are ignored during the subsetting process.

For more information, go to Identifying lookup tables.

Related tables are tables that are connected by direct or indirect relationships with a target table, and that are not identified as lookup tables.

  • Downstream tables have data that is required to maintain referential integrity for the subset. These tables have primary keys that are referenced by foreign keys in related tables.

  • Upstream tables contain data that has a foreign key that references a primary key in the target table. For large upstream tables, if the foreign key columns are not indexed, the subsetting process can be significantly slower. These upstream records are not required to maintain referential integrity, but can contain useful information. In the subset configuration, you can filter these upstream records either by date or by using a WHERE clause.

Some related tables are both downstream and upstream. In that case, you can provide a filter that applies only to the upstream records. Because the downstream records are required for referential integrity, they cannot be filtered.

For example, a transactions table contains a foreign key column to identify the customer. The value is the primary key of a record in the customers table. The customers table is downstream of the transactions table - the transaction data is incomplete without the customer information. The transactions table is upstream of the customers table.

Structural pulls data from related tables in order to preserve referential integrity in the output data subset.

In many cases, the relationship is direct. For example, a target table contains a list of events. The events table identifies the user that hosted the event. The user is identified using a foreign key relationship from the events table to the users table. The users table is a related table. The subset includes all the users that the events refer to.

The relationship also might be indirect. To continue the example, the events table identifies a user from the users table. The users table identifies the company that each user belongs to. The company is identified using a foreign key relationship from the users table to the companies table. The companies table is also a related table. The subset needs to include all of the companies that are referred to by the users that the events table refers to.

For an example of how Structural identifies related tables, view the example diagram in How Structural creates a subset.

Out-of-subset tables

Tables other than target tables, lookup tables, or related tables are not part of the subset.

By default, Structural copies only the table schema of out-of-subset tables. It does not populate any of the data.

You can also choose to process the tables using the table mode that is assigned to each table.

For more information, go to Determining how to process tables that are not in the subset.

How Structural creates a subset

Structural creates the subset before it applies any transformations to the source data.

To provide a basic overview of how Structural creates the subset, we'll use the following simple example schema:

The Events table is the target table for the subset. The Events table includes information about the event hosts (Hosts table) and the event venue (Venues table). For each host, the data includes the company that the host belongs to (Companies table).

The Attendees table includes the event that the attendee registered for.

The Hosts, Companies, Venues, and Attendees tables are all related tables for the subset.

The States table provides a lookup of state values to use for the company, venue, and attendee addresses. It is a lookup table for the subset. A subset always includes all of the data in a lookup table.

When you enable subsetting for a data generation job:

  1. To create the basis of the subset, Structural gets data from the target tables based on the configured filters, either a percentage or a WHERE clause. In our example, Structural gets the subset of data from the Events table. Structural then traverses your database based on the relationships that originate from the target tables.

  2. Structural first goes upstream. For the upstream process, Structural traverses through tables that reference a target table, based on the data collected in step 1. In other words, the value of the primary key for a target table record is the value of a foreign key column in the upstream table. This step continues until there are no remaining upstream tables to process. To continue our example, Structural retrieves the attendees for the event records that are in the subset.

  3. Next, Structural goes downstream. Structural traverses all of the tables to look for foreign key columns for which the value is the primary key of an upstream table record. To continue our example, Structural retrieves the hosts and venues that are referred to in the event records that it retrieved in the first or second pass on the events table. It also retrieves the companies that are referred to in the host records. During this downstream step, Structural considers both upstream and downstream tables to ensure that the subset includes every connected table. For example, if the Venues table included a foreign key column that referenced a primary key from the Attendees table, Tonic would have to return to the Attendees table to get those attendee records.

Other notes about retrieving subset data

You might want to be aware of how Structural retrieves subset data in the following cases, which can result in either more or less data than you might expect.

If there are multiple target tables, and the tables are related to each other, Structural takes the union of the required data for both the target table configuration and the table relationships.

For example, table A contains a foreign key column that refers to table B. You configure both tables as target tables. For table B, Structural pulls both the directly targeted set of records, and the records that the targeted table A records refer to.

Tables that are upstream of multiple target tables

If a table is upstream of multiple target tables, then Structural only pulls records from that table that contain references to targeted records in all of the target tables.

For example, in related table Child1, column1 is a foreign key that refers to a primary key in target table Parent1. column2 is a foreign key that refers to a primary key in target table Parent2.

If column1 and column2 both refer to targeted records in Parent1 and Parent2, then that Child1 record is included in the subset. If only one of those columns refers to a targeted record in Parent1 or Parent2, then that Child1 record is not included.

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