Skip expression columns when validating index for compression

When enabling compression on a hypertable, we check the unique indexes
defined on it to provide performance improvement hints in case the
unique index columns are not specified as segmentby columns as well.
However this check threw an error when expression columns were present
in the index, preventing the user from enabling compression.
This patch fixes this by simply ignoring the expression columns in the
index, since we cannot currently segment by an expression.

Fixes #6205

.unreleased/PR_6209

@@ -0,0 +1 @@
+Fixes: #6209 Allow enabling compression on hypertable with unique expression index

tsl/src/compression/create.c

@@ -956,6 +956,8 @@ validate_existing_indexes(Hypertable *ht, CompressColInfo *colinfo, Bitmapset *i
 		for (int i = 0; i < index->indnkeyatts; i++)
 		{
 			int attno = index->indkey.values[i];
+			if (attno == 0)
+				continue; /* skip check for expression column */
 			Form_hypertable_compression col_def = get_col_info_for_attnum(ht, colinfo, attno);
 			Ensure(col_def, "missing column definition for unique index");
 			if (col_def->segmentby_column_index < 1 && col_def->orderby_column_index < 1)

tsl/test/expected/compression_ddl.out

@@ -2392,3 +2392,20 @@ EXPLAIN (COSTS OFF) SELECT * FROM space_part ORDER BY time;
                      ->  Seq Scan on _hyper_36_135_chunk
 (35 rows)
 
+-- test creation of unique expression index does not interfere with enabling compression
+-- github issue 6205
+create table mytab (col1 varchar(100) not null, col2 integer not null,
+value double precision not null default 0, arrival_ts timestamptz not null, departure_ts timestamptz not null
+default current_timestamp);
+select create_hypertable('mytab', 'departure_ts');
+WARNING:  column type "character varying" used for "col1" does not follow best practices
+  create_hypertable  
+---------------------
+ (38,public,mytab,t)
+(1 row)
+
+create unique index myidx_unique ON
+mytab (lower(col1::text), col2, departure_ts, arrival_ts);
+alter table mytab set (timescaledb.compress);
+WARNING:  column "col2" should be used for segmenting or ordering
+WARNING:  column "arrival_ts" should be used for segmenting or ordering

tsl/test/sql/compression_ddl.sql

@@ -965,3 +965,14 @@ EXPLAIN (COSTS OFF) SELECT * FROM space_part ORDER BY time;
 INSERT INTO space_part VALUES
 ('2022-01-01 00:02', 1, 1, 1);
 EXPLAIN (COSTS OFF) SELECT * FROM space_part ORDER BY time;
+
+-- test creation of unique expression index does not interfere with enabling compression
+-- github issue 6205
+create table mytab (col1 varchar(100) not null, col2 integer not null,
+value double precision not null default 0, arrival_ts timestamptz not null, departure_ts timestamptz not null
+default current_timestamp);
+
+select create_hypertable('mytab', 'departure_ts');
+create unique index myidx_unique ON
+mytab (lower(col1::text), col2, departure_ts, arrival_ts);
+alter table mytab set (timescaledb.compress);