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Qualify predicates chapter's index, sort-order, and function-availability claims #49

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thiagooak merged 1 commit into from
Sep 21, 2026
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Qualify predicates chapter's index, sort-order, and function-availability claims #49

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14 changes: 10 additions & 4 deletions resources/chapters/predicates.edn
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Original file line number Diff line number Diff line change
Expand Up @@ -34,7 +34,10 @@
"The next chapter puts it to use."]
[:p "Predicates work on strings too, this site allows "
[:code "clojure.string/starts-with?"] " and " [:code "clojure.string/includes?"]
"; in a real Datomic app, any Clojure function is available. "
". In a real Datomic app, any pure " [:code "clojure.core"] " function is available (except "
[:code "eval"] "), along with any Java or Clojure function on the classpath of the process "
"that runs the query. With the Client API that process is the server, not your application, "
"so your own functions aren't available unless they are deployed there too. "
"Here we find Pokemon whose name starts with \"S\":"]
[:ui/runnable "pokemon"
[:find ?name
Expand All @@ -44,14 +47,17 @@
[:p [:strong "Good to know: "] [:code "="] ", " [:code "!="] ", " [:code "<"] ", " [:code "<="] ", "
[:code ">"] " and " [:code ">="] " are "
[:a {:href "https://docs.datomic.com/query/query-data-reference.html#range-predicates"} "range predicates"]
". Datomic answers them with an index instead of checking every value, "
". Datomic answers them with an index instead of checking every value "
"(on Datomic Pro this only applies to attributes marked " [:code ":db/index true"] " or "
[:code ":db/unique"] "; Datomic Cloud and Local, which this site runs on, index every attribute already), "
"so they are much more efficient than the same test written with another function. "
"Datomic's own "
[:a {:href "https://docs.datomic.com/query/query-data-reference.html#built-in-predicates-and-functions"} "built-in functions"]
" are also cheaper than your own, since they run inside the query engine. "
"Our database is too small to notice, but this matters in a real one. "
"Strings sort in dictionary order, so the names starting with \"S\" are also the names from \"S\" up to, "
"but not including, \"T\". That is the same "
"Strings sort by character code, not dictionary order, so uppercase comes before lowercase. "
"Every name here is capitalized, which is why the names starting with \"S\" are also the names "
"from \"S\" up to, but not including, \"T\". That is the same "
[:ui/value "pokemon" [:find (count ?e) . :where [?e :pokemon/name ?name] [(clojure.string/starts-with? ?name "S")]]]
" Pokemon as before:"]
[:ui/runnable "pokemon"
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