I have a unique constraint on a character varying column that will mostly have lengths of approximately 600 but could get to 10 million.

I have read about B-Trees, but I can't determine how much disk space the index will consume or if such large-sized values will prevent the index from functioning performantly.

What are the impacts upon disk space and time of a B-Tree index on character varying columns with such potentially large lengths?

1 Answer 1


In Postgres, a unique constraint is implemented with a unique B-tree index. The manual:

Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns used in the constraint.

Indexes use the same basic storing mechanisms as tables: arrays of data pages. Indexes have some minor additional features. The manual:

In a table, an item is a row; in an index, an item is an index entry.

Every table and index is stored as an array of pages of a fixed size (usually 8 kB, although a different page size can be selected when compiling the server).

The maximum size of an index entry is roughly a third of a data page. See:

The maximum size was reduced to 2704 bytes in Postgres 12. Still the same in Postgres 15. Meaning your UNIQUE constraint is not possible.

Consider a unique index on hash value, either after adding a redundant column with the hash, or just an index on the hash expression. You could use the built-in function md5() or something more efficient for huge values. Related:

  • Thank you Erwin Brandstetter, incredible advice as usual! For performance, should I use bytea? If so, should I let Postgres do the conversions with md5 and decode or sha512() with this library zedwood.com/article/cpp-sha512-function? I'm fairly certain I'll never need to extract the hash, but I will need to compare with external data. Thank you so very much for sharing your great knowledge!
    – user32234
    Commented Jun 27, 2014 at 1:04
  • @Gracchus: I suggest you put that in new questions (one topic per piece). Comments are not the place. You can always link to this one for reference. Commented Jun 27, 2014 at 1:16
  • Thank you Erwin Brandstetter! I have posted per your recommendation: dba.stackexchange.com/questions/69170/…
    – user32234
    Commented Jun 27, 2014 at 1:45
  • 3
    I wonder if something has changed in newer Postgres version, as I can insert text values with 10000 (random) characters into a column using a unique index: rextester.com/ZDYS66763
    – user1822
    Commented Sep 5, 2018 at 12:41
  • 2
    @a_horse_with_no_name: Interesting, indeed. Worthy of a new question: dba.stackexchange.com/q/217087/3684 (I think I found the answer.) Commented Sep 8, 2018 at 16:22

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