Merging two existing plain tables can be more efficient than indexing the data from scratch and desired in some cases (such as merging 'main' and 'delta' tables instead of simply rebuilding 'main' in the 'main+delta' partitioning scheme). So indexer has an option to do that. Merging tables is normally faster than rebuilding, but still not instant on huge tables. Basically, it will need to read the contents of the both tables once and write the result once. Merging 100 GB and 1 GB table, for example, will result in 202 GB of I/O (but that's still likely less than the indexing from scratch requires).

The basic command syntax is as follows:

sudo -u manticore indexer --merge DSTINDEX SRCINDEX [--rotate] [--drop-src]

Unless --drop-src is specified only the DSTINDEX table will be affected: the contents of SRCINDEX will be merged into it.

--rotate switch will be required if DSTINDEX is already being served by searchd.

The typical usage pattern is to merge a smaller update from SRCINDEX into DSTINDEX. Thus, when merging attributes the values from SRCINDEX will win if duplicate document IDs are encountered. Note, however, that the "old" keywords will not be automatically removed in such cases. For example, if there's a keyword "old" associated with document 123 in DSTINDEX, and a keyword "new" associated with it in SRCINDEX, document 123 will be found by both keywords after the merge. You can supply an explicit condition to remove documents from DSTINDEX to mitigate that; the relevant switch is --merge-dst-range:

sudo -u manticore indexer --merge main delta --merge-dst-range deleted 0 0

This switch lets you apply filters to the destination table along with merging. There can be several filters; all of their conditions must be met in order to include the document in the resulting merged table. In the example above, the filter passes only those records where 'deleted' is 0, eliminating all records that were flagged as deleted.

--drop-src allows dropping SRCINDEX after the merge and before rotating the tables, which is important in case you specify DSTINDEX in killlist_target of DSTINDEX, otherwise when rotating the tables the documents that have been merged into DSTINDEX may be suppressed by SRCINDEX.