Detail the constraints in applying OPTIMIZE

Document the fact that partitions having
only one data file without any delete files
are disregarded for `OPTIMIZE` scenarios
in case of the Iceberg connector.

Author: findinpath

docs/src/main/sphinx/connector/iceberg.md

@@ -802,7 +802,48 @@ created before the partitioning change.
 The connector supports the following commands for use with {ref}`ALTER TABLE
 EXECUTE <alter-table-execute>`.
 
-```{include} optimize.fragment
+##### optimize
+
+The `optimize` command is used for rewriting the content of the specified
+table so that it is merged into fewer but larger files. If the table is
+partitioned, the data compaction acts separately on each partition selected for
+optimization. This operation improves read performance.
+
+All files with a size below the optional `file_size_threshold` parameter
+(default value for the threshold is `100MB`) are merged in case any of the
+following conditions per partition / table (for unpartitioned tables)
+are met:
+
+-  more than one data file to merge is present
+-  at least one data file, with delete files attached, is present
+
+```sql
+ALTER TABLE test_table EXECUTE optimize
+```
+
+The following statement merges files in a table that are
+under 128 megabytes in size:
+
+```sql
+ALTER TABLE test_table EXECUTE optimize(file_size_threshold => '128MB')
+```
+
+You can use a `WHERE` clause with the columns used to partition the table
+to filter which partitions are optimized:
+
+```sql
+ALTER TABLE test_partitioned_table EXECUTE optimize
+WHERE partition_key = 1
+```
+
+You can use a more complex `WHERE` clause to narrow down the scope of the
+`optimize` procedure. The following example casts the timestamp values to
+dates, and uses a comparison to only optimize partitions with data from the year
+2022 or newer:
+
+```
+ALTER TABLE test_table EXECUTE optimize
+WHERE CAST(timestamp_tz AS DATE) > DATE '2021-12-31'
 ```
 
 Use a `WHERE` clause with [metadata columns](iceberg-metadata-columns) to filter