1. A computer program product comprising a machine-readable medium storing instructions that, when executed by at least one programmable processor, cause the at least one programmable processor to perform operations comprising: storing, in a plurality of main part fragments, a plurality of data records that comprise a data set, each main part fragment comprising a subset of the set of data records; assigning a relative age to each main part fragment of the plurality of main part fragments; creating, for a newest of the main part fragments, a fragment-specific index segment, the fragment-specific index segment providing a lookup ability for logical identifiers of data records in just the newest of the main part fragments; and providing, for two or more older main fragments, a multi-fragment index segment that spans the two or more older main fragments, the multi-fragment index segment providing a lookup ability for logical identifiers of data records in the two or more older main part fragments.