JanKaul commented on code in PR #11041:
URL: https://github.com/apache/iceberg/pull/11041#discussion_r4094119181
##########
format/view-spec.md:
##########
@@ -322,3 +449,198 @@
s3://bucket/warehouse/default.db/event_agg/metadata/00002-(uuid).metadata.json
} ]
}
```
+
+### Materialized View Example
+
+Imagine the following operation, which creates a materialized view that
precomputes daily event counts:
+
+```sql
+USE prod.default
+```
+```sql
+CREATE MATERIALIZED VIEW event_agg_mv (
+ event_count COMMENT 'Count of events',
+ event_date)
+COMMENT 'Precomputed daily event counts'
+AS
+SELECT
+ COUNT(1), CAST(event_ts AS DATE)
+FROM events
+GROUP BY 2
+```
+
+The materialized view metadata JSON file looks as follows:
+
+```
+s3://bucket/warehouse/default.db/event_agg_mv/metadata/00001-(uuid).metadata.json
+```
+```json
+{
+ "view-uuid": "b2a12651-3038-4a72-8a31-5027ab84da35",
+ "format-version" : 1,
+ "location" : "s3://bucket/warehouse/default.db/event_agg_mv",
+ "current-version-id" : 1,
+ "properties" : {
+ "comment" : "Precomputed daily event counts"
+ },
+ "versions" : [ {
+ "version-id" : 1,
+ "timestamp-ms" : 1573518431292,
+ "schema-id" : 1,
+ "default-catalog" : "prod",
+ "default-namespace" : [ "default" ],
+ "summary" : {
+ "engine-name" : "Spark",
+ "engine-version" : "3.4.1"
+ },
+ "representations" : [ {
+ "type" : "sql",
+ "sql" : "SELECT\n COUNT(1), CAST(event_ts AS DATE)\nFROM
events\nGROUP BY 2",
+ "dialect" : "spark"
+ } ],
+ "storage-table" : {
+ "namespace" : [ "default" ],
+ "name" : "event_agg_mv__storage"
+ }
+ } ],
+ "schemas": [ {
+ "schema-id": 1,
+ "type" : "struct",
+ "fields" : [ {
+ "id" : 1,
+ "name" : "event_count",
+ "required" : false,
+ "type" : "int",
+ "doc" : "Count of events"
+ }, {
+ "id" : 2,
+ "name" : "event_date",
+ "required" : false,
+ "type" : "date"
+ } ]
+ } ],
+ "version-log" : [ {
+ "timestamp-ms" : 1573518431292,
+ "version-id" : 1
+ } ]
+}
+```
+
+After a refresh operation, the storage table's snapshot summary contains the
`refresh-state` property.
+The following is an example of the `refresh-state` JSON value stored in the
snapshot summary of the storage table:
+
+```json
+{
+ "view-version-id" : 1,
Review Comment:
Agreed the MV is genuinely stale once the comment is added — I'd keep the
freshness rule strict on view-version-id. But this is where a "smart" REFRESH
MATERIALIZED VIEW helps: the refresh can detect that only a column comment
changed and that the rest of the version and the precomputed data are still
valid, then write a new refresh-state with the updated view-version-id without
recomputing the data. The MV is fresh again after that. The tradeoff is that
users may need to run a REFRESH in such cases to update the metadata — which
avoids having to define/parse semantic SQL equivalence in the spec (your
reconciliation layer's job) on the consumer side.
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