Delta encoding - Amazon Redshift
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Delta encoding

Delta encodings are very useful for date time columns.

Delta encoding compresses data by recording the difference between values that follow each other in the column. This difference is recorded in a separate dictionary for each block of column values on disk. (An Amazon Redshift disk block occupies 1 MB.) For example, suppose that the column contains 10 integers in sequence from 1 to 10. The first are stored as a 4-byte integer (plus a 1-byte flag). The next nine are each stored as a byte with the value 1, indicating that it is one greater than the previous value.

Delta encoding comes in two variations:

  • DELTA records the differences as 1-byte values (8-bit integers)

  • DELTA32K records differences as 2-byte values (16-bit integers)

If most of the values in the column could be compressed by using a single byte, the 1-byte variation is very effective. However, if the deltas are larger, this encoding, in the worst case, is somewhat less effective than storing the uncompressed data. Similar logic applies to the 16-bit version.

If the difference between two values exceeds the 1-byte range (DELTA) or 2-byte range (DELTA32K), the full original value is stored, with a leading 1-byte flag. The 1-byte range is from -127 to 127, and the 2-byte range is from -32K to 32K.

The following table shows how a delta encoding works for a numeric column.

Original data value Original size (bytes) Difference (delta) Compressed value Compressed size (bytes)
1 4 1 1+4 (flag + actual value)
5 4 4 4 1
50 4 45 45 1
200 4 150 150 1+4 (flag + actual value)
185 4 -15 -15 1
220 4 35 35 1
221 4 1 1 1
Totals 28 15