Appending to and overwriting a Delta Lake table

This section explains how to append to an exising Delta table and how to overwrite a Delta table.

Delta Lake append transactions

Suppose you have a Delta table with the following contents:

+-------+----------+
|   num | letter   |
|-------+----------|
|     1 | a        |
|     2 | b        |
|     3 | c        |
+-------+----------+

Append two additional rows of data to the table:

from deltalake import write_deltalake, DeltaTable

df = pd.DataFrame({"num": [8, 9], "letter": ["dd", "ee"]})
write_deltalake("tmp/some-table", df, mode="append")

Here are the updated contents of the Delta table:

+-------+----------+
|   num | letter   |
|-------+----------|
|     1 | a        |
|     2 | b        |
|     3 | c        |
|     8 | dd       |
|     9 | ee       |
+-------+----------+

Now let's see how to perform an overwrite transaction.

Delta Lake overwrite transactions

Now let's see how to overwrite the exisitng Delta table.

df = pd.DataFrame({"num": [11, 22], "letter": ["aa", "bb"]})
write_deltalake("tmp/some-table", df, mode="overwrite")

Here are the contents of the Delta table after the overwrite operation:

+-------+----------+
|   num | letter   |
|-------+----------|
|    11 | aa       |
|    22 | bb       |
+-------+----------+

Overwriting just performs a logical delete. It doesn't physically remove the previous data from storage. Time travel back to the previous version to confirm that the old version of the table is still accessable.

dt = DeltaTable("tmp/some-table", version=1)

+-------+----------+
|   num | letter   |
|-------+----------|
|     1 | a        |
|     2 | b        |
|     3 | c        |
|     8 | dd       |
|     9 | ee       |
+-------+----------+