The Teradata Optimizer cannot determine the number of defined partitions directly from the table's Data Definition Language (DDL) when the partitioning expression is based on the CASEN function. The CASEN function contains conditional logic that is opaque to the Optimizer, preventing it from mapping column values to specific partitions without external information. In the absence of collected statistics on the partitioning columns, the Optimizer makes a conservative, worst-case assumption that the table has the maximum number of partitions (65,535 for 2-byte partitioning). This can lead to the selection of a suboptimal query plan due to overestimated costs.

A. Partitioning on a character column does not inherently cause this assumption; the complexity of the function used on the column is the cause.

C. The number or data type of partitioning columns is not the direct cause; the function used in the partitioning expression is the determining factor.

D. While RANGEN can also cause this behavior, CASEN represents a more general and arbitrary conditional logic that the Optimizer cannot parse without statistics.

1. Teradata Vantage™ - Database Design, Release 17.20 (B035-1094-172K), Chapter 10: Partitioned Primary Indexes, Section: "Using CASEN for Row Partitioning". The manual states, "Without statistics on the partitioning columns, the Optimizer assumes the maximum number of partitions (65,535 for 2-byte partitioning) and might not be able to generate a good query plan."

2. Teradata Vantage™ SQL Data Definition Language Detailed Topics, Release 17.20 (B035-1184-172K), Chapter 5: CREATE TABLE, Section: "PARTITION BY Clause". This document notes, "If you do not collect statistics on the partitioning column set for a table partitioned using a CASEN or RANGEN function, the Optimizer assumes that the table has the maximum number of partitions possible for its partition level..."

3. Teradata Vantage™ - SQL Request and Transaction Processing, Release 17.20 (B035-1142-172K), Chapter 11: Optimizer, Section: "Accessing Partitioned Primary Index Tables". This reference confirms, "For partitioning expressions that use the CASEN or RANGEN functions, the Optimizer cannot determine the number of partitions from the table definition. If statistics have not been collected... the Optimizer assumes the maximum number of partitions (65,535 for 2-byte partitioning)."