What is the most efficient way to store tags in a database?

I am implementing a tagging system on my website similar to one stackoverflow uses, my question is - what is the most effective way to store tags so that they may be searched and filtered?

My idea is this:

Table: Items
Columns: Item_ID, Title, Content

Table: Tags
Columns: Title, Item_ID

Is this too slow? Is there a better way?

One item is going to have many tags. And one tag will belong to many items. This implies to me that you'll quite possibly need an intermediary table to overcome the many-to-many obstacle.

Something like:

>Table: Items
Columns: Item_ID, Item_Title, Content

>Table: Tags
Columns: Tag_ID, Tag_Title

>Table: Items_Tags
Columns: Item_ID, Tag_ID

It might be that your web app is very very popular and need de-normalizing down the road, but it's pointless muddying the waters too early.

Actually I believe de-normalising the tags table might be a better way forward, depending on scale.

This way, the tags table simply has tagid, itemid, tagname.

You'll get duplicate tagnames, but it makes adding/removing/editing tags for specific items MUCH more simple. You don't have to create a new tag, remove the allocation of the old one and re-allocate a new one, you just edit the tagname.

For displaying a list of tags, you simply use DISTINCT or GROUP BY, and of course you can count how many times a tag is used easily, too.

If you don't mind using a bit of non-standard stuff, Postgres version 9.4 and up has an option of storing a record of type JSON text array.

Your schema would be:

Table: Items
Columns: Item_ID:int, Title:text, Content:text

Table: Tags
Columns: Item_ID:int, Tag_Title:text[]

For more info, see this excellent post by Josh Berkus: http://www.databasesoup.com/2015/01/tag-all-things.html

There are more various options compared thoroughly for performance and the one suggested above is the best overall.

You can't really talk about slowness based on the data you provided in a question. And I don't think you should even worry too much about performance at this stage of developement. It's called premature optimization.

However, I'd suggest that you'd include Tag_ID column in the Tags table. It's usually a good practice that every table has an ID column.

I'd suggest using intermediary third table for storing tags<=>items associations, since we have many-to-many relations between tags and items, i.e. one item can be associated with multiple tags and one tag can be associated with multiple items. HTH, Valve.

If space is going to be an issue, have a 3rd table Tags(Tag_Id, Title) to store the text for the tag and then change your Tags table to be (Tag_Id, Item_Id). Those two values should provide a unique composite primary key as well.

Items should have an "ID" field, and Tags should have an "ID" field (Primary Key, Clustered).

Then make an intermediate table of ItemID/TagID and put the "Perfect Index" on there.

Tag Schema: Tag tables and attributes:

Tables:

tags (each row only keeps information about a particular tag)
taggings (each row keeps information about trigger and who will receive the trigger )
products_tags (each row keeps information about tag with particular product)
tag_status (each row keeps track of a tag status)

Table: tags Attributes of tags table:

id(PK)
userId(FK users)(not null)(A tag only belongs to one user, but  a user can create multiple tags. So it is one to many relationships.)
genreId(FK products_geners)(not null)
name (string) (not null)
description (string)
status (int) (0=inactive, 1=pending, 2=active, there could be more flag)
rank(int)  (rank is the popularity of a particular tag), this field can be use for sorting among similar tags.)
type (int) (0=type1, 1=type2, 2=type3)
photo(string)
visibility (int) (0=public, 2=protected, 3 = private)(private means the tag only  visible to assigned users of a product, protected means a tag only visible to all friends and followers of the creator of the tag, public means search by public, such as all admin created tag)
createdAt(timestamp for the tag was created at)
updatedAt (timestamp for the tag last time updated)
deletedAt (default value null) (timestamp when tag was deleted, we need this field because we will delete tag permanently from audit table). 

Note: Keeping field no 10 will come handy later.

Table: taggings :

This table will be used for triggering such as broadcasting other users' feed or sending them notification. After a row inserted in this table, there will be a service who will read a row take associated action to remove the row.

Attributes of taggings table:

Id(PK)
tagId(a tagging row only belongs to a tag, but a tag can have multiple row).
taggableId (id of a user who will receive notification)
taggableType(int) (0=notification, 1=feed message)
taggerId(the person who triggered the broadcast)
taggerType(ad, product, news)
createdAt(timestamp for the tag was created at)

Table: products_tags

From user perspective a user able to create a tag after instantiating an product, so bellow table will keep information about which products has which tags.

Attributes of Attributes of taggings table:

Id (PK)
productId(FK)
tagId(FK)

Table: tag_status

When user will create a tag, a row will be created in this table with tagId and default status inactive/pending, admin will pull all tags from tags table where status=pending/inactive, after reviewing a tag if admin approved the tag then value of status in tag table will be approved and the row of tag_status will be removed. If admin is rejected then the value of the status field of tag_status table will be rejected and a trigger will be broadcasted and the receiver will send a notification to the associated user of that tag with a message that his tag is rejected .

id(PK)
senderId(Id of the user)
receiverId(Id of admin user)
createdAt(timestamp of created at)
updatedAt(timestamp of updated at)
deletedAt(timestamp of deletedAt) default value null
expiredAt (if a tag never gets approved it will expire after a certain time for removing its information from the database. If a rejected tag gets updated by user then expiredAt will reset to new future time)
status 
Message (string varchar(256)) (message for user)