docs(notes): add ibm-db2-indexes-en notes

Author: lzwjava

Diff for: notes/2025-04-09-ibm-db2-indexes-en.md

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+---
+title: SQL Query to List Indexes in IBM Db2
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+In IBM Db2, you can query the system catalog views to find information about a table's existing indexes. The primary catalog view for indexes is `SYSCAT.INDEXES`. This view contains details about all indexes in the database, including the table they belong to, the columns they index, and their properties (e.g., unique or non-unique).
+
+Here’s how you can check the existing indexes for a specific table:
+
+### SQL Query to List Indexes
+Use the following query to retrieve index information for a specific table:
+
+```sql
+SELECT 
+    INDNAME, 
+    TABSCHEMA, 
+    TABNAME, 
+    COLNAMES, 
+    UNIQUERULE, 
+    INDEXTYPE
+FROM 
+    SYSCAT.INDEXES
+WHERE 
+    TABNAME = 'YOUR_TABLE_NAME'
+    AND TABSCHEMA = 'YOUR_SCHEMA_NAME';
+```
+
+#### Explanation of Columns
+- **`INDNAME`**: The name of the index.
+- **`TABSCHEMA`**: The schema of the table.
+- **`TABNAME`**: The name of the table.
+- **`COLNAMES`**: The columns included in the index (prefixed with `+` for ascending or `-` for descending).
+- **`UNIQUERULE`**: Indicates if the index is unique (`U` for unique, `D` for duplicate-allowed, `P` for primary key).
+- **`INDEXTYPE`**: The type of index (e.g., `REG` for regular, `CLUS` for clustering).
+
+#### Example
+To find the indexes for a table named `EMPLOYEES` in the schema `MY_SCHEMA`:
+```sql
+SELECT 
+    INDNAME, 
+    TABSCHEMA, 
+    TABNAME, 
+    COLNAMES, 
+    UNIQUERULE, 
+    INDEXTYPE
+FROM 
+    SYSCAT.INDEXES
+WHERE 
+    TABNAME = 'EMPLOYEES'
+    AND TABSCHEMA = 'MY_SCHEMA';
+```
+
+### Notes
+1. **Case Sensitivity**: Table and schema names in Db2 are typically uppercase in the catalog unless explicitly created with quotes (e.g., `"employees"`). So, use `'EMPLOYEES'` instead of `'employees'` unless you’re sure of the casing.
+2. **Schema**: If you don’t specify `TABSCHEMA`, you might get results for tables with the same name in different schemas. To find your schema, you can check your current schema with:
+   ```sql
+   SELECT CURRENT SCHEMA FROM SYSIBM.SYSDUMMY1;
+   ```
+3. **Permissions**: You need appropriate privileges to query `SYSCAT.INDEXES`. Typically, `SELECT` access on the catalog views is granted to most users.
+
+### Alternative: Using `DESCRIBE`
+For a quick check in some Db2 environments (e.g., Db2 LUW), you can use the `DESCRIBE` command to see indexes, though it’s less detailed:
+```sql
+DESCRIBE INDEXES FOR TABLE schema_name.table_name SHOW DETAIL;
+```
+
+#### Example
+```sql
+DESCRIBE INDEXES FOR TABLE MY_SCHEMA.EMPLOYEES SHOW DETAIL;
+```
+
+This command lists indexes but might not be available or as comprehensive in all Db2 editions (e.g., Db2 on z/OS).
+
+### Sample Output
+For the query on `SYSCAT.INDEXES`, you might see something like:
+```
+INDNAME        TABSCHEMA  TABNAME    COLNAMES       UNIQUERULE  INDEXTYPE
+-------------  ---------- ---------- -------------- ----------- ---------
+EMP_ID_IDX     MY_SCHEMA  EMPLOYEES  +EMPLOYEE_ID   U           REG
+EMP_NAME_IDX   MY_SCHEMA  EMPLOYEES  +LAST_NAME     D           REG
+```
+- `EMP_ID_IDX` is a unique index on `EMPLOYEE_ID`.
+- `EMP_NAME_IDX` is a non-unique index on `LAST_NAME`.
+
+### If You Want All Indexes in the Database
+Omit the `WHERE` clause:
+```sql
+SELECT 
+    INDNAME, 
+    TABSCHEMA, 
+    TABNAME, 
+    COLNAMES, 
+    UNIQUERULE, 
+    INDEXTYPE
+FROM 
+    SYSCAT.INDEXES
+ORDER BY 
+    TABSCHEMA, TABNAME, INDNAME;
+```
+
+Let me know if you need help interpreting the results or refining the query!