Let's put the loading concepts we've discussed into practice. By working through a few common scenarios, you'll get a better feel for how to choose the right loading strategy and anticipate potential challenges. Remember, the goal of the loading stage is to reliably get your transformed data into the target system where it can be used.
Imagine your team is building a brand-new data warehouse to analyze sales performance. You have several years' worth of historical sales data extracted from an old system and transformed into clean CSV files. The target table in the data warehouse, let's call it FactSales
, is currently completely empty.
FactSales
table and your transformed CSV data?Think it through: Since the FactSales
table is empty and you want to populate it with all the historical data you have prepared, completely filling the table is the objective. An incremental load wouldn't make sense because there's nothing existing to update or add to.
Answer: A Full Load (Lfull) is the appropriate strategy. It's designed to load an entire dataset, often involving truncating (emptying) the target table first (though in this case, it's already empty) and then inserting all the transformed records.
Before loading, it's essential to:
FactSales
table structure (columns, data types, constraints) matches the structure of your transformed data.FactSales
table.The initial load was successful! Now, your data warehouse contains historical sales. Going forward, each morning, your ETL process extracts only the sales transactions from the previous day. This new data needs to be added to the FactSales
table without affecting the existing historical records.
Think it through: Performing a full load every day would be highly inefficient. It would involve deleting all existing data and reloading everything, including the history that hasn't changed, just to add one day's worth of transactions. You only want to add the new records.
Answer: An Incremental Load (Lincremental) is the correct approach. Specifically, since these are entirely new transactions for the previous day, an Append strategy (a type of incremental load) is most suitable. You are simply adding new rows to the FactSales
table.
To prevent duplicates, you need a way to identify the records already loaded. This typically involves tracking the load date or using the transaction dates themselves. Your ETL process needs logic to ensure it only processes records for the specific previous day it's meant to handle.
Your data warehouse also includes a DimProduct
table containing details about each product sold (ID, name, category, price). Occasionally, information about existing products changes in the source system (e.g., a price update, a category change). Your ETL process extracts a file containing only the products with changes, identified by their unique ProductID
.
DimProduct
table if a ProductID
from the incoming file matches an existing product? What if the ProductID
is entirely new (which might indicate a data quality issue or a genuinely new product)?Think it through: Like the daily sales, you don't want to reload the entire product dimension table every time a few prices change. However, unlike the daily sales, you aren't just adding new records; you need to modify existing ones.
Answer: This calls for an Incremental Load (Lincremental), but specifically one that handles Updates (often called an "Upsert" or "Merge" operation).
ProductID
matches an existing row in DimProduct
, the loading process should update that row with the new information (e.g., the new price).ProductID
does not exist in DimProduct
, you need a defined business rule. Common approaches are:
This update/insert logic is a common pattern in dimension table loading.
Let's revisit the daily sales load (Scenario 2). The FactSales
table requires dates in YYYY-MM-DD
format. However, during today's load, a few rows in the source file mistakenly have dates formatted as MM/DD/YYYY
. Your transformation logic didn't catch this.
'05/20/2024'
into a date column expecting '2024-05-20'
?Think it through: Databases and data warehouses enforce data types. Trying to insert text that doesn't conform to the target column's expected format (like putting 'MM/DD/YYYY'
into a strict DATE type) will usually cause a problem.
Answer: The loading process will likely fail for those specific rows. The database will reject the insert because the provided string '05/20/2024'
cannot be implicitly converted to the required YYYY-MM-DD
date format.
How the overall load behaves depends on its configuration:
For beginner pipelines, logging errors and deciding whether to stop or continue loading good records is a common requirement. Ideally, data formatting issues like this should be caught and corrected during the Transformation stage, before attempting the load.
Imagine your transformed data looks like this (simplified):
transaction_id
, sale_timestamp
, product_identifier
, units_sold
, revenue_usd
Your target table FactSales
has this structure:
SalesKey
(auto-incrementing integer), TransactionID
(varchar), DateKey
(integer, links to DimDate table), ProductKey
(integer, links to DimProduct table), QuantitySold
(integer), Revenue
(decimal)Assume your transformation stage already handled converting sale_timestamp
to a DateKey
and product_identifier
to a ProductKey
through lookups.
A diagram showing the mapping from source fields to target table columns.
SalesKey
is generated by the database, whileDateKey
andProductKey
are assumed to be derived during transformation before loading.
Mapping:
transaction_id
-> TransactionID
sale_timestamp
(transformed) -> DateKey
product_identifier
(transformed) -> ProductKey
units_sold
-> QuantitySold
revenue_usd
-> Revenue
SalesKey
is typically generated automatically by the database upon insertion, so it doesn't require a direct mapping from the source file.By working through these scenarios, you can see how understanding the nature of your data (initial vs. ongoing, new vs. updates) and your target system is fundamental to choosing the correct loading strategy and anticipating the necessary steps like schema mapping and error handling.
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