feat: support SHOW DEFAULT_TRANSACTION_ISOLATION for PG databases

The `SHOW DEFAULT_TRANSACTION_ISOLATION` command can be used in PostgreSQL to query
the current default isolation level for a connection. This command was not supported
by the Spanner JDBC driver and PGAdapter, which made it difficult for customers to
check whether their attempt to set a different default isolation level actually worked.

Updates GoogleCloudPlatform/pgadapter#3984

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionStatementExecutor.java

@@ -152,6 +152,8 @@ StatementResult statementSetPgSessionCharacteristicsTransactionMode(
 
   StatementResult statementShowTransactionIsolationLevel();
 
+  StatementResult statementShowDefaultTransactionIsolation();
+
   StatementResult statementSetProtoDescriptors(byte[] protoDescriptors);
 
   StatementResult statementSetProtoDescriptorsFilePath(String filePath);

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionStatementExecutorImpl.java

@@ -64,6 +64,7 @@
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_COMMIT_RESPONSE;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_COMMIT_TIMESTAMP;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_DATA_BOOST_ENABLED;
+import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_DEFAULT_TRANSACTION_ISOLATION;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_DELAY_TRANSACTION_START_UNTIL_FIRST_WRITE;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_DIRECTED_READ;
 import static com.google.cloud.spanner.connection.StatementResult.ClientSideStatementType.SHOW_EXCLUDE_TXN_FROM_CHANGE_STREAMS;
@@ -647,7 +648,19 @@ public StatementResult statementShowReadLockMode() {
 
   @Override
   public StatementResult statementShowTransactionIsolationLevel() {
-    return resultSet("transaction_isolation", "serializable", SHOW_TRANSACTION_ISOLATION_LEVEL);
+    TransactionOptions.IsolationLevel isolationLevel = getConnection().getDefaultIsolationLevel();
+    if (getConnection().isInTransaction()) {
+      isolationLevel = getConnection().getTransactionIsolationLevel();
+    }
+    return resultSet("transaction_isolation", isolationLevel, SHOW_TRANSACTION_ISOLATION_LEVEL);
+  }
+
+  @Override
+  public StatementResult statementShowDefaultTransactionIsolation() {
+    return resultSet(
+        "default_transaction_isolation",
+        getConnection().getDefaultIsolationLevel(),
+        SHOW_DEFAULT_TRANSACTION_ISOLATION);
   }
 
   @Override

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/StatementResult.java

@@ -98,6 +98,7 @@ enum ClientSideStatementType {
     SET_RPC_PRIORITY,
     SHOW_RPC_PRIORITY,
     SHOW_TRANSACTION_ISOLATION_LEVEL,
+    SHOW_DEFAULT_TRANSACTION_ISOLATION,
     SHOW_SAVEPOINT_SUPPORT,
     SET_SAVEPOINT_SUPPORT,
     SHOW_DATA_BOOST_ENABLED,

google-cloud-spanner/src/main/resources/com/google/cloud/spanner/connection/PG_ClientSideStatements.json

@@ -248,6 +248,15 @@
       "method": "statementShowTransactionIsolationLevel",
       "exampleStatements": ["show transaction isolation level","show variable transaction isolation level"]
     },
+    {
+      "name": "SHOW [VARIABLE] DEFAULT_TRANSACTION_ISOLATION",
+      "executorName": "ClientSideStatementNoParamExecutor",
+      "resultType": "RESULT_SET",
+      "statementType": "SHOW_DEFAULT_TRANSACTION_ISOLATION",
+      "regex": "(?is)\\A\\s*show\\s+(?:variable\\s+)?default_transaction_isolation\\s*\\z",
+      "method": "statementShowDefaultTransactionIsolation",
+      "exampleStatements": ["show default_transaction_isolation","show variable default_transaction_isolation"]
+    },
     {
       "name": "EXPLAIN <sql>",
       "executorName": "ClientSideStatementExplainExecutor",