Schema change: tested on PostgreSQL 12 to 18, and a row count fix for 12 and 13

server/src/schema-change/catalog.ts

@@ -1,3 +1,4 @@
+import { unknownRows } from "./row-estimate.js";
 import { runReadOnly, type Queryable } from "../services/pg-read.js";
 import { USER_SCHEMAS } from "../services/pg-relations.js";
 
@@ -55,9 +56,9 @@ const TABLES_SQL = `
   SELECT n.nspname AS schema, c.relname AS name, c.relkind = 'p' AS partitioned,
          CASE WHEN c.relkind = 'p' THEN (SELECT count(*) FROM pg_partition_tree(c.oid) t WHERE t.level > 0) ELSE 0 END AS partition_count,
          CASE WHEN c.relkind = 'p'
-              THEN (SELECT CASE WHEN bool_or(l.reltuples < 0) THEN NULL ELSE sum(l.reltuples)::int8::text END
+              THEN (SELECT CASE WHEN bool_or(${unknownRows("l")}) THEN NULL ELSE sum(l.reltuples)::int8::text END
                       FROM pg_partition_tree(c.oid) t JOIN pg_class l ON l.oid = t.relid WHERE t.isleaf)
-              WHEN c.reltuples < 0 THEN NULL ELSE c.reltuples::int8::text END AS estimated_rows,
+              WHEN ${unknownRows("c")} THEN NULL ELSE c.reltuples::int8::text END AS estimated_rows,
          CASE WHEN c.relkind = 'p'
               THEN (SELECT COALESCE(sum(pg_total_relation_size(t.relid)), 0) FROM pg_partition_tree(c.oid) t WHERE t.isleaf)::text
               ELSE pg_total_relation_size(c.oid)::text END AS total_bytes,

server/src/schema-change/facts.ts

@@ -1,3 +1,4 @@
+import { unknownRows } from "./row-estimate.js";
 import { runReadOnly, type Queryable } from "../services/pg-read.js";
 import { quoteLiteral } from "../services/pg-sql.js";
 
@@ -182,8 +183,8 @@ export async function loadFacts(client: Queryable, qualified: string[]): Promise
         `${cte}
          SELECT t.oid::int8::text AS oid, t.schema, t.name, t.relkind = 'p' AS partitioned,
                 CASE WHEN t.relkind = 'p'
-                     THEN (SELECT CASE WHEN bool_or(l.reltuples < 0) THEN NULL ELSE sum(l.reltuples)::int8::text END FROM pg_partition_tree(t.oid) p JOIN pg_class l ON l.oid = p.relid WHERE p.isleaf)
-                     ELSE (SELECT CASE WHEN c.reltuples < 0 THEN NULL ELSE c.reltuples::int8::text END FROM pg_class c WHERE c.oid = t.oid) END AS estimated_rows,
+                     THEN (SELECT CASE WHEN bool_or(${unknownRows("l")}) THEN NULL ELSE sum(l.reltuples)::int8::text END FROM pg_partition_tree(t.oid) p JOIN pg_class l ON l.oid = p.relid WHERE p.isleaf)
+                     ELSE (SELECT CASE WHEN ${unknownRows("c")} THEN NULL ELSE c.reltuples::int8::text END FROM pg_class c WHERE c.oid = t.oid) END AS estimated_rows,
                 CASE WHEN t.relkind = 'p' THEN (SELECT COALESCE(sum(pg_relation_size(p.relid)), 0) FROM pg_partition_tree(t.oid) p WHERE p.isleaf)::text ELSE pg_relation_size(t.oid)::text END AS heap_bytes,
                 CASE WHEN t.relkind = 'p' THEN (SELECT COALESCE(sum(pg_indexes_size(p.relid)), 0) FROM pg_partition_tree(t.oid) p WHERE p.isleaf)::text ELSE pg_indexes_size(t.oid)::text END AS index_bytes,
                 CASE WHEN t.relkind = 'p' THEN (SELECT COALESCE(sum(pg_total_relation_size(p.relid)), 0) FROM pg_partition_tree(t.oid) p WHERE p.isleaf)::text ELSE pg_total_relation_size(t.oid)::text END AS total_bytes,

server/src/schema-change/health.ts

@@ -1,3 +1,4 @@
+import { unknownRows } from "./row-estimate.js";
 import { runReadOnly, type Queryable } from "../services/pg-read.js";
 import { USER_SCHEMAS } from "../services/pg-relations.js";
 
@@ -161,7 +162,7 @@ const TABLES_SQL = `
   SELECT n.nspname AS schema, c.relname AS name, c.relkind = 'p' AS partitioned,
          (SELECT count(*) FROM pg_attribute a WHERE a.attrelid = c.oid AND a.attnum > 0 AND NOT a.attisdropped)::int AS columns,
          EXISTS (SELECT 1 FROM pg_index ix WHERE ix.indrelid = c.oid AND ix.indisprimary) AS has_pk,
-         CASE WHEN c.relkind = 'p' THEN NULL WHEN c.reltuples < 0 THEN NULL ELSE c.reltuples::int8::text END AS estimated_rows,
+         CASE WHEN c.relkind = 'p' THEN NULL WHEN ${unknownRows("c")} THEN NULL ELSE c.reltuples::int8::text END AS estimated_rows,
          CASE WHEN c.relkind = 'p' THEN (SELECT COALESCE(sum(pg_total_relation_size(p.relid)), 0) FROM pg_partition_tree(c.oid) p WHERE p.isleaf)::text ELSE pg_total_relation_size(c.oid)::text END AS total_bytes,
          s.n_live_tup::text AS live, s.n_dead_tup::text AS dead, s.n_mod_since_analyze::text AS mods,
          GREATEST(s.last_analyze, s.last_autoanalyze) AS last_analyze, (s.relid IS NOT NULL) AS has_stats_row

server/src/schema-change/row-estimate.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from "vitest";
+import { unknownRows } from "./row-estimate.js";
+
+describe("unknownRows", () => {
+    it("reads -1 as unknown on every version, and 0 on a table with data only before PostgreSQL 14", () => {
+        const sql = unknownRows("c");
+        expect(sql).toContain("c.reltuples < 0");
+        expect(sql).toContain("c.reltuples = 0 AND current_setting('server_version_num')::int < 140000 AND pg_relation_size(c.oid) > 0");
+    });
+
+    it("uses the alias it is given", () => {
+        expect(unknownRows("leaf")).toContain("leaf.reltuples");
+        expect(unknownRows("leaf")).toContain("pg_relation_size(leaf.oid)");
+        expect(unknownRows("leaf")).not.toMatch(/\bc\./);
+    });
+});

server/src/schema-change/row-estimate.ts

@@ -0,0 +1,8 @@
+/**
+ * SQL for "this table's row count is not known". PostgreSQL 14 and later store -1 in pg_class.reltuples for a table that has never been
+ * vacuumed or analyzed. Before that the same table reports 0, which cannot be told from an empty table by reltuples alone: a table that
+ * holds data (its file is not empty) but reports 0 has simply not been analyzed. Reading that 0 as "empty" would let the compiler take
+ * the shortcuts that are only safe for empty tables on a table of any size.
+ */
+export const unknownRows = (alias: string): string =>
+    `(${alias}.reltuples < 0 OR (${alias}.reltuples = 0 AND current_setting('server_version_num')::int < 140000 AND pg_relation_size(${alias}.oid) > 0))`;