Advanced Query Question: Find page with 2 specific blocks

[mentaloid]

@naught101

so a couple of questions

More than a couple. Should definitely read much more and practice a lot.

• Nobody grasped the syntax just by guessing.
• The advanced queries doc is merely a starting point.
• Should experiment with simpler queries in tiny test-graphs.

I’m going to answer your individual points in an unprecise but simplified way:

1. All parts that look like ?name are arbitrarily-named variables.
   • They don’t relate to the page content.
     • They apply to a database maintained by Logseq separately from the markdown files.
   • They are not built-in, so there cannot be a list of them.
     • The names are chosen by the users, the computer doesn’t care.
   • Same-named variables represent:
     • the same single value at any given time
     • but potentially multiple values during the same execution of the same query
   • They are not defined, they are placeholders.
2. The :where clauses:
   • combine:
     • with an implicit top-level AND
     • sequentially from top to bottom
   • apply before the :find clause
     • They apply to the entries of the database.
       • The first clause filters the whole database, the second clause filters the results of the first clause and so on.
     • Technically speaking, all :find, :keys and :where are at the same level.
       • However, they apply in this order: :where → :find → :keys
3. Easier reading.
4. It is sequential.
   • The same results can be achieved in different line-order (and also in multiple ways).
   • Different orders (and/or ways) can differ in their performance.

Here is a rough but thorough description of what goes on at a middle level:

• Begin with :where
  • Gather (the IDs of) all blocks which have properties.
    • This is equivalent to filtering-out all blocks without properties.
    • Name those blocks ?details and their properties ?details-props
      • At this point, variable ?details holds a relatively big number of IDs.
  • Then keep only those ?details-blocks that have property material::
    • And name the value ?material
    • At this point, variable ?details holds less IDs than in the beginning.
  • Then keep only if that property’s value is “Aluminium”.
    • This clause cannot be placed earlier than where the variable ?material appears.
    • At this point, variable ?details holds even less IDs than in the beginning.
  • Then gather all (the IDs of) the pages that contain the blocks with ?details-IDs.
    • Name those pages ?page
    • At this point, variable ?page holds relatively many IDs.
      • But not more than the number of ?details-blocks.
  • Then keep only the ?page-pages that have blocks.
    • They all have, because they already contain ?details-blocks.
    • We still need to do this step, in order to consider all other blocks in ?page-pages.
      • Name those blocks ?summary
        • At this point:
          • variable ?summary holds a relatively big number of IDs.
          • ?details-blocks is a subset of ?summary-blocks
            • because although they both belong to the same ?page-pages:
              • ?details-blocks got further filtered by property
              • while ?summary-blocks are not filtered yet
    • Alternatively, it is possible to:
      • begin with ?summary-blocks and deal with ?details-blocks later
        • The relative performance of that approach depends on the actual graph.
      • move the two [?... :block/page ?page] clauses to the bottom
        • That would be less performant.
      • even swap the two [?... :block/page ?page] clauses with each-other
        • That would be worse, though fully valid.
  • Then from ?page-pages keep only the ones that contain blocks with properties.
    • They all do, because they already contain ?details-blocks.
    • We still need to do this step, in order to consider all other blocks with properties.
      • Name those properties ?summary-props
        • At this point, ?details-props is a subset of ?summary-props
  • Then keep only those that have property type::
    • And name the value ?type
    • At this point:
      • variable ?summary holds less IDs than earlier
      • thus variable ?page holds fewer IDs than earlier
      • thus variable ?details also shrinks (if it was to be used again)
  • Then keep only if that property’s value is “Product”.
    • This clause cannot be placed earlier than where the variable ?type appears.
    • At this point:
      • variable ?summary holds even less IDs (if it was to be used again)
      • thus variable ?page holds much fewer IDs
      • thus variable ?details shrinks further (if it was to be used again)
• Continue with :find
  • Return all the fields of the remaining ?page-pages.
  • If no pages remain, return nothing.