There are a few limitations on the data which can be stored in an index; these rarely get in the way, and are generally easy to work around if they do, but it’s worth knowing about them.
Terms are limited to 245 bytes in length (at least with the “glass” and “chert” backends), but each zero byte in a term is currently internally encoded as two bytes, so the limit is less for a term which contains zero bytes. It’s rarely useful to have longer terms, but one situation where it can be is if you’re using something like a URL as an ID term; there is some discussion of this as one of our FAQs.
Document data length¶
The document data has a length limit which depends on the blocksize and some other factors, but with the default block size of 8KB, the document data length limit will be somewhere over 100MB.
Document value length¶
Document values are limited in length to a similar length to document data, but for performance reasons you probably wouldn’t want to store document values longer than a few tens of bytes, as reading multiple 100MB+ values during the match would be rather slow.
Document IDs are (currently) 32-bit by default which limits you to 232-1 (nearly 4.3 billion) documents in a database. Document IDs for deleted documents aren’t reused for when automatically assigning a new document ID, so this limit also includes documents you’ve deleted. You can effectively reclaim such no-longer-used document IDs by compacting the database.
If you configure xapian-core with –enable-64bit-docid then 64-bit docids will be used instead. You may well also want to make termcounts 64-bit with –enabl-64bit-termcount. Note that these options change type sizes and hence the ABI of the library.
B-tree block number¶
The B-trees use a 32-bit unsigned block count. The default blocksize is 8KB which limits you to 32TB tables. You can increase the blocksize if this is a problem, but it’s best to do it before you create the database as otherwise you need to use xapian-compact to make a compacted copy of the database with the new blocksize, and that will take a while for such a large database. The maximum blocksize currently allowed is 64K, which limits you to 256TB tables.
OS file size¶
Any operating or filing system limit on file size obviously applies to Xapian. On modern platforms, you’re unlikely to hit these limits (e.g. on Linux, ext4 allows files up to 16TB and filesystems up to 1EB, while btrfs allows files and filesystems up to 16EB (figures from Wikipedia).
Total document length limit¶
Glass stores the total length (i.e. number of terms) of all the documents in a database so it can calculate the average document length. This is currently stored as an unsigned 64-bit quantity so you’re almost certain to hit another limit first.