diff --git a/src/main/kotlin/dev/usbharu/hideout/application/infrastructure/exposed/ExposedTransaction.kt b/src/main/kotlin/dev/usbharu/hideout/application/infrastructure/exposed/ExposedTransaction.kt
index 4dc8316b..3438d428 100644
--- a/src/main/kotlin/dev/usbharu/hideout/application/infrastructure/exposed/ExposedTransaction.kt
+++ b/src/main/kotlin/dev/usbharu/hideout/application/infrastructure/exposed/ExposedTransaction.kt
@@ -4,12 +4,11 @@ import dev.usbharu.hideout.application.external.Transaction
 import kotlinx.coroutines.slf4j.MDCContext
 import org.jetbrains.exposed.sql.transactions.experimental.newSuspendedTransaction
 import org.springframework.stereotype.Service
-import java.sql.Connection
 
 @Service
 class ExposedTransaction : Transaction {
     override suspend fun <T> transaction(block: suspend () -> T): T {
-        return newSuspendedTransaction(MDCContext(), transactionIsolation = Connection.TRANSACTION_SERIALIZABLE) {
+        return newSuspendedTransaction(MDCContext()) {
             block()
         }
     }
diff --git a/src/main/resources/application.yml b/src/main/resources/application.yml
index c9c2604f..daff34db 100644
--- a/src/main/resources/application.yml
+++ b/src/main/resources/application.yml
@@ -18,8 +18,6 @@ spring:
       WRITE_DATES_AS_TIMESTAMPS: false
     default-property-inclusion: always
   datasource:
-    hikari:
-      transaction-isolation: "TRANSACTION_SERIALIZABLE"
     driver-class-name: org.postgresql.Driver
     url: "jdbc:postgresql:hideout2"
     username: "postgres"
diff --git a/src/main/resources/db/migration/V1__Init_DB.sql b/src/main/resources/db/migration/V1__Init_DB.sql
index 15a61994..4ea80255 100644
--- a/src/main/resources/db/migration/V1__Init_DB.sql
+++ b/src/main/resources/db/migration/V1__Init_DB.sql
@@ -3,9 +3,9 @@ create table if not exists instance
     id              bigint primary key,
     "name"          varchar(1000)  not null,
     description     varchar(5000)  not null,
-    url             varchar(255)   not null,
+    url          varchar(255) not null unique,
     icon_url        varchar(255)   not null,
-    shared_inbox    varchar(255)   null,
+    shared_inbox varchar(255) null unique,
     software        varchar(255)   not null,
     version         varchar(255)   not null,
     is_blocked      boolean        not null,
@@ -21,9 +21,9 @@ create table if not exists users
     screen_name varchar(300)   not null,
     description varchar(10000) not null,
     password    varchar(255)   null,
-    inbox       varchar(1000)  not null,
-    outbox      varchar(1000)  not null,
-    url         varchar(1000)  not null,
+    inbox  varchar(1000) not null unique,
+    outbox varchar(1000) not null unique,
+    url    varchar(1000) not null unique,
     public_key  varchar(10000) not null,
     private_key varchar(10000) null,
     created_at  bigint         not null,
@@ -31,6 +31,7 @@ create table if not exists users
     "following" varchar(1000)  null,
     followers   varchar(1000)  null,
     "instance"  bigint         null,
+    unique (name, domain),
     constraint fk_users_instance__id foreign key ("instance") references instance (id) on delete restrict on update restrict
 );
 create table if not exists follow_requests
@@ -73,7 +74,7 @@ create table if not exists posts
     repost_id   bigint                null,
     reply_id    bigint                null,
     "sensitive" boolean default false not null,
-    ap_id       varchar(100)          not null
+    ap_id varchar(100) not null unique
 );
 alter table posts
     add constraint fk_posts_userid__id foreign key (user_id) references users (id) on delete restrict on update restrict;