feat: replace sequences query with empty table (#366)

* Replacing sequences querying to empty table as sequences is not supported in spanner. * Varchar initialization.

Author: pratickchokhani

src/main/java/com/google/cloud/spanner/pgadapter/statements/JdbcMetadataStatementHelper.java

@@ -36,7 +36,7 @@ class JdbcMetadataStatementHelper {
  static boolean isPotentialJdbcMetadataStatement(String sql) {
  // All JDBC metadata queries that need any replacements reference the pg_catalog schema or the
  // pg_settings table.
- return sql.contains("pg_catalog.") || sql.contains("pg_settings");
+ return sql.contains("pg_catalog.") || sql.contains("pg_settings") || sql.contains("pg_class");
  }
 
  static String replaceJdbcMetadataStatement(String sql) {
@@ -146,6 +146,9 @@ static String replaceJdbcMetadataStatement(String sql) {
  if (sql.startsWith(PgJdbcCatalog.PG_JDBC_GET_TYPE_INFO_SIMPLE_PREFIX_42_3)) {
  return PgJdbcCatalog.PG_JDBC_GET_TYPE_INFO_REPLACEMENT_42_3;
  }
+ if (sql.startsWith(PgJdbcCatalog.PG_JDBC_GET_SEQUENCES)) {
+ return PgJdbcCatalog.PG_JDBC_GET_SEQUENCES_REPLACEMENT;
+ }
 
  // The query that is used to fetch the primary keys of a table was re-written when the PG driver
  // went from 42.2.x to 42.3.x. We translate the 42.3.x query back to the 42.2.x version here,

src/main/java/com/google/cloud/spanner/pgadapter/utils/PgJdbcCatalog.java

@@ -965,4 +965,9 @@ public class PgJdbcCatalog {
  + "inner join pg_catalog.pg_class rel on a.attrelid=rel.oid\n"
  + "inner join pg_catalog.pg_namespace n on rel.relnamespace=n.oid\n"
  + "inner join information_schema.columns c on n.nspname=c.table_schema and rel.relname=c.table_name and a.attname=c.column_name\n";
+
+ public static final String PG_JDBC_GET_SEQUENCES =
+ "select relname from pg_class where relkind='S'";
+ public static final String PG_JDBC_GET_SEQUENCES_REPLACEMENT =
+ "select ''::varchar as relname LIMIT 0";
 }