pvary commented on a change in pull request #2826:
URL: https://github.com/apache/hive/pull/2826#discussion_r759369978
##########
File path:
standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/metastore/ObjectStore.java
##########
@@ -5265,6 +5265,54 @@ private void copyMSD(MStorageDescriptor newSd,
MStorageDescriptor oldSd) {
oldSd.setStoredAsSubDirectories(newSd.isStoredAsSubDirectories());
}
+ /**
+ * Checks if a column descriptor has any remaining references by storage
descriptors
+ * in the db.
+ * @param oldCD the column descriptor to check if it has references or not
+ * @return true if has references
+ */
+ private boolean hasRemainingCDReference(MColumnDescriptor oldCD) {
+ assert oldCD != null;
+ Query query = null;
+
+ /**
+ * In order to workaround oracle not supporting limit statement caused
performance issue, HIVE-9447 makes
+ * all the backend DB run select count(1) from SDS where SDS.CD_ID=? to
check if the specific CD_ID is
+ * referenced in SDS table before drop a partition. This select count(1)
statement does not scale well in
+ * Postgres, and there is no index for CD_ID column in SDS table.
+ * For a SDS table with with 1.5 million rows, select count(1) has average
700ms without index, while in
+ * 10-20ms with index. But the statement before
+ * HIVE-9447( SELECT * FROM "SDS" "A0" WHERE "A0"."CD_ID" = $1 limit 1)
uses less than 10ms .
+ */
+ try {
+ // HIVE-21075: Fix Postgres performance regression caused by HIVE-9447
+ DatabaseProduct dbProduct =
DatabaseProduct.determineDatabaseProduct(MetaStoreDirectSql.getProductName(pm),
conf);
+ if (dbProduct.isPOSTGRES() || dbProduct.isMYSQL()) {
+ query = pm.newQuery(MStorageDescriptor.class, "this.cd == inCD");
+ query.declareParameters("MColumnDescriptor inCD");
+ List<MStorageDescriptor> referencedSDs =
listStorageDescriptorsWithCD(oldCD, query);
+ //if no other SD references this CD, we can throw it out.
+ if (referencedSDs != null && referencedSDs.isEmpty()) {
+ return false;
+ }
+ } else {
+ query = pm.newQuery(
+ "select count(1) from
org.apache.hadoop.hive.metastore.model.MStorageDescriptor where (this.cd ==
inCD)");
+ query.declareParameters("MColumnDescriptor inCD");
+ long count = (Long) query.execute(oldCD);
+ //if no other SD references this CD, we can throw it out.
+ if (count == 0) {
+ return false;
+ }
+ }
+ return true;
Review comment:
Nit: For me it would be more natural to do this here:
```
return count != 0;
```
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