Add Get Row Count For Most Recent Query as a Postgres TIL

postgres/get-row-count-for-most-recent-query.md

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+# Get Row Count For Most Recent Query
+
+Anytime you execute a query in `psql`, there is a _row count_ associated with
+that query. This is most naturally understood with a `select` query where a
+discreet number of rows are returned. We typically see the row count (e.g. `(19
+rows)`) right below the result set.
+
+You can always reference the row count of the most recent query with [the
+`:ROW_COUNT`
+variable](https://www.postgresql.org/docs/current/app-psql.html#APP-PSQL-VARIABLES-ROW-COUNT).
+Here we use `\echo` to print it out.
+
+```sql
+> select generate_series(2,20);
+ generate_series
+-----------------
+               2
+               3
+               4
+               5
+               6
+               7
+               8
+               9
+              10
+              11
+              12
+              13
+              14
+              15
+              16
+              17
+              18
+              19
+              20
+(19 rows)
+
+Time: 12.338 ms
+> \echo :ROW_COUNT
+19
+```
+
+For some queries, like one that induces a pager (e.g. `less`) to be used,
+you'll lose track of the row count once the pager closes. This is where being
+able to reference the row count without rerunning the query is most useful.
+
+```sql
+> select generate_series(2,2000);
+Time: 9.815 ms
+> \echo :ROW_COUNT
+1999
+```
+
+Notice, we can also get a row count from other kinds of queries like this
+`insert` statement.
+
+```sql
+> insert into users (id) values (50001), (50002), (50003);
+INSERT 0 3
+Time: 2.804 ms
+> \echo :ROW_COUNT
+3
+```