In DB2 EXPLAIN output, what does it mean when the value of MATCHCOLS is 0 for an access path?

Introduction / Context:
DB2 provides an EXPLAIN facility that reveals how the optimizer plans to execute a query. One important piece of information in the access path is MATCHCOLS, which indicates how many leading columns of an index are matched by the query predicates. Understanding the meaning of MATCHCOLS helps database professionals diagnose performance problems and tune indexes. This question focuses on the special case where MATCHCOLS is 0.

Given Data / Assumptions:
DB2 tables often have indexes defined on one or more columns.Query predicates may or may not match the indexed columns in a way that allows efficient index based access.The EXPLAIN output includes a MATCHCOLS value for each index access path.MATCHCOLS represents the number of index key columns that are used for matching predicates.

Concept / Approach:
When MATCHCOLS is greater than zero, it means that some leading columns of the index key participate in search predicates, allowing DB2 to position itself efficiently within the index. For example, MATCHCOLS equal to 2 on a three column index suggests that the first two key columns are used for matching. When MATCHCOLS is 0, none of the index key columns are used in matching predicates. In practice, this usually implies that DB2 cannot use the index to directly locate qualifying rows and may resort to a full table scan or a non matching index scan, where it reads many index entries without narrow positioning.

Step-by-Step Solution:
First, recall that indexes are useful when query predicates restrict values on the indexed columns, allowing the optimizer to rapidly locate matching rows.Next, understand that MATCHCOLS counts how many leading key columns are effectively used in this matching process.Then, consider what happens if MATCHCOLS is 0. In that case, no index key column is used to filter the data, so the index does not help narrow down the search based on predicates.After that, realize that DB2 may treat such an access path similarly to a table scan or non matching index scan, reading many rows or entries before applying predicates.Finally, compare this behavior with the options and select option A, which states that no columns match and that the access is typically a scan rather than an efficient index match.

Verification / Alternative check:
Database tuning references for DB2 explain that MATCHCOLS indicates how many index columns are used to match predicates in the WHERE clause. They note that when MATCHCOLS is zero, the index is not being used to match rows based on predicate values, and performance may be similar to a scan. These sources emphasize that higher MATCHCOLS values are usually better for selective access. There is no guidance that MATCHCOLS 0 means perfect matching, empty tables, or error free execution status, which supports option A as the correct interpretation.

Why Other Options Are Wrong:
Option B reverses the meaning by claiming that MATCHCOLS 0 means all columns match, which contradicts the definition. Option C states that DB2 refuses to execute queries with MATCHCOLS 0, which is not true; the query still runs, but perhaps less efficiently. Option D attributes the value to empty tables, while table emptiness and MATCHCOLS are independent concepts. Option E treats MATCHCOLS as a status flag for success or failure, which is not its purpose; MATCHCOLS describes index column matching, not execution status.

Common Pitfalls:
A common pitfall is ignoring EXPLAIN output and MATCHCOLS when investigating slow queries. Developers may add indexes but write predicates that do not align with the leading columns, resulting in MATCHCOLS 0 even though an index exists. Another pitfall is misinterpreting MATCHCOLS as a count of columns in the table rather than a measure of predicate matching. Best practice is to design indexes that match common query predicates and to verify through EXPLAIN that MATCHCOLS is positive and meaningful for critical queries. Recognizing the implications of MATCHCOLS 0 is a key step in this tuning process.

Final Answer:
The correct answer is: No columns in the predicate match the index key, so the index is not used for matching and the access is typically a table scan or a non matching index scan.