SQL explain plan: what is Materialize?

sql postgresql sql-execution-plan
Claudiu picture Claudiu · Jun 12, 2010 · Viewed 26.4k times · Source

I asked PostgreSQL to explain my query. Part of the explanation was:

table_name --> Materialize

What does materialize do? I'm joining two tables, not views or anything like that.

Answer

Magnus Hagander picture Magnus Hagander · Jun 13, 2010

A materialize node means the output of whatever is below it in the tree (which can be a scan, or a full set of joins or something like that) is materalized into memory before the upper node is executed. This is usually done when the outer node needs a source that it can re-scan for some reason or other.

So in your case, the planner is determining that the result of a scan on one of your tables will fit in memory, and it till make it possible to choose an upper join operation that requires rescans while still being cheaper.

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