Tom Lane wrote: > Bruce Momjian <[EMAIL PROTECTED]> writes: > > * Consider ways of storing rows more compactly on disk > > o Support a smaller header for short variable-length fields? > > With respect to the business of having different on-disk and in-memory > representations, we have that already today: see TOAST. It strikes me > that it might be useful to think about solving the problem with a > "second generation toast mechanism". The first generation only worried > about storing large values, but the second generation would also address > the problem of storing small values efficiently. > > Or you could think about it as a "second generation varlena". This > mindset would make for a slightly different set of choices about where > the work gets done. I'm not sure which is better. > > Either way, I think it would be interesting to consider > > (a) length word either one or two bytes, not four. You can't need more > than 2 bytes for a datum that fits in a disk page ...
That is an interesting observation, though could compressed inline values exceed two bytes? > (b) alignment either one or two bytes, not four. TEXT would be > perfectly happy with 1-byte alignment, but for NUMERIC we might want 2. > > I'm inclined to bag the idea of storing the length words separately from > the data proper. Although it probably would make for some marginal gain > in cache efficiency, I don't see any reasonable way at all to fit it > into the current system structure, whereas either the "toast" or "next > gen varlena" approaches seem fairly straightforward. And having to > track an additional pointer inside the inner loops of heap_form_tuple > and heap_deform_tuple could eat up any performance gain anyway. Good point. How do we do it now? I assume we store just the fixed-size toast pointer length in the heap attribute, not the toast length. Why haven't we investigated shrinking the varlena header before? -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
