Postgresql 如何在9.5级考试中正确地进行upsert

postgresql 9.5 upsert的正确语法，下面的查询显示column reference "gallery_id" is ambiguous个错误，为什么？

var dbQuery = `INSERT INTO category_gallery (
  category_id, gallery_id, create_date, create_by_user_id
  ) VALUES ($1, $2, $3, $4)
  ON CONFLICT (category_id)
  DO UPDATE SET
  category_id = $1,
  last_modified_date = $3,
  last_modified_by_user_id = $4
  WHERE gallery_id = $2`;

我try 将WHERE gallery_id = $2;改为WHERE category_gallery.gallery_id = $2;，然后显示错误there is no unique or exclusion constraint matching the ON CONFLICT specification，但I don't want to set gallery_id or category_id as unique因为我想确保两列相同，然后进行更新....

如何正确地在postgres 9.5中进行upsert？

如果ON CONFLICT需要唯一列，我应该使用其他方法吗，如何？

我想确定多列都冲突，然后做更新，什么是正确的用法

var dbQuery = `INSERT INTO category_gallery (
  category_id, gallery_id, create_date, create_by_user_id
  ) VALUES ($1, $2, $3, $4)
  ON CONFLICT (category_id, gallery_id)
  DO UPDATE SET
  category_id = $1,
  last_modified_date = $3,
  last_modified_by_user_id = $4
  WHERE gallery_id = $2`;


var dbQuery = `INSERT INTO category_gallery (
  category_id, gallery_id, create_date, create_by_user_id
  ) VALUES ($1, $2, $3, $4)
  ON CONFLICT (category_id AND gallery_id)
  DO UPDATE SET
  category_id = $1,
  last_modified_date = $3,
  last_modified_by_user_id = $4
  WHERE gallery_id = $2`;

表(类别id、 gallery id非唯一列)

category_id | gallery_id | create_date | create_by_user_id | last_modified_date | last_modified_by_user_id
1 | 1 | ...  
1 | 2 | ...
2 | 2 | ...
1 | 3 | ...