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--- documentation:tutorials:json_query [2018/11/26 12:27] – Remove unnecessary "nowiki" tag from within code tag rjs7
+++ documentation:tutorials:json_query [2022/02/10 13:34] (current) – external edit 127.0.0.1
@@ Line 2094: / Line 2094: @@
+==== Choosing the Order of JOINs ====
+As of Evergreen 3.0.2, we support user-defined join order in cstore and friends. Previously, because the join structure of ''oils_sql'' beyond the specification of a single table was only allowed to be represented as a JSON object, it was subject to potential hash key reordering. By supporting an intervening array layer, one can now specify the exact join order of the tables in a join tree.
+For example, given the following JSON object passing through a modern Perl 5 interpreter as a nested hash:
+<code json>
+{
+  select : {
+    acp: ['id'],
+    acn: ['record'],
+    acpl: ['name']
+  },
+  from : {
+    acp: {
+      acn: {filter: {record: 12345} },
+      acpl: null
+    }
+  }
+}
+</code>
+the FROM clause of the query may end up as:
+<code sql>
+FROM acp
+    JOIN acn ON (acp.call_number = acn.id AND acn.record = 12345)
+    JOIN acpl ON (acp.location = acpl.id)
+</code>
+Or as:
+<code sql>
+FROM acp
+    JOIN acpl ON (acp.location = acpl.id)
+    JOIN acn ON (acp.call_number = acn.id AND acn.record = 12345)
+</code>
+In some situations, the join order will matter either to the semantics of the query plan, or to its performance. The following example of the newly supported syntax illustrates how to specify join order:
+<code json>
+{
+  select : {
+    acp: ['id'],
+    acn: ['record'],
+    acpl: ['name']
+  },
+  from : {
+    acp: [
+      {acn: {filter: {record: 12345} } },
+      'acpl'
+    ]
+  }
+}
+</code>
+And the only FROM clause the can be generated is:
+<code sql>
+FROM acp
+    JOIN acn ON (acp.call_number = acn.id AND acn.record = 12345)
+    JOIN acpl ON (acp.location = acpl.id)
+</code>
 ==== Things You Can't Do ====