Allow COPY FROM to filter data using WHERE conditions

Extends the COPY FROM command with a WHERE condition, which allows doing
various types of filtering while importing the data (random sampling,
condition on a data column, etc.).  Until now such filtering required
either preprocessing of the input data, or importing all data and then
filtering in the database. COPY FROM ... WHERE is an easy-to-use and
low-overhead alternative for most simple cases.

Author: Surafel Temesgen
Reviewed-by: Tomas Vondra, Masahiko Sawada, Lim Myungkyu
Discussion: https://www.postgresql.org/message-id/flat/CALAY4q_DdpWDuB5-Zyi-oTtO2uSk8pmy+dupiRe3AvAc++1imA@mail.gmail.com

Author: tvondra

doc/src/sgml/ref/copy.sgml

@@ -25,6 +25,7 @@ PostgreSQL documentation
 COPY <replaceable class="parameter">table_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ]
     FROM { '<replaceable class="parameter">filename</replaceable>' | PROGRAM '<replaceable class="parameter">command</replaceable>' | STDIN }
     [ [ WITH ] ( <replaceable class="parameter">option</replaceable> [, ...] ) ]
+    [ WHERE <replaceable class="parameter">condition</replaceable> ]
 
 COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ] | ( <replaceable class="parameter">query</replaceable> ) }
     TO { '<replaceable class="parameter">filename</replaceable>' | PROGRAM '<replaceable class="parameter">command</replaceable>' | STDOUT }
@@ -353,6 +354,32 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>WHERE</literal></term>
+    <listitem>
+   <para>
+    The optional <literal>WHERE</literal> clause has the general form
+<synopsis>
+WHERE <replaceable class="parameter">condition</replaceable>
+</synopsis>
+    where <replaceable class="parameter">condition</replaceable> is
+    any expression that evaluates to a result of type
+    <type>boolean</type>.  Any row that does not satisfy this
+    condition will not be inserted to the table.  A row satisfies the
+    condition if it returns true when the actual row values are
+    substituted for any variable references.
+   </para>
+
+   <para>
+    Currently, subqueries are not allowed in <literal>WHERE</literal>
+    expressions, and the evaluation does not see any changes made by the
+    <command>COPY</command> itself (this matters when the expression
+    contains calls to <literal>VOLATILE</literal> functions).
+   </para>
+
+    </listitem>
+   </varlistentry>
+
   </variablelist>
  </refsect1>
 

src/backend/commands/copy.c

@@ -39,7 +39,11 @@
 #include "miscadmin.h"
 #include "optimizer/clauses.h"
 #include "optimizer/planner.h"
+#include "optimizer/prep.h"
 #include "nodes/makefuncs.h"
+#include "parser/parse_coerce.h"
+#include "parser/parse_collate.h"
+#include "parser/parse_expr.h"
 #include "parser/parse_relation.h"
 #include "port/pg_bswap.h"
 #include "rewrite/rewriteHandler.h"
@@ -149,6 +153,7 @@ typedef struct CopyStateData
 	bool		convert_selectively;	/* do selective binary conversion? */
 	List	   *convert_select; /* list of column names (can be NIL) */
 	bool	   *convert_select_flags;	/* per-column CSV/TEXT CS flags */
+	Node	   *whereClause;	/* WHERE condition (or NULL) */
 
 	/* these are just for error messages, see CopyFromErrorCallback */
 	const char *cur_relname;	/* table name for error messages */
@@ -179,6 +184,7 @@ typedef struct CopyStateData
 	ExprState **defexprs;		/* array of default att expressions */
 	bool		volatile_defexprs;	/* is any of defexprs volatile? */
 	List	   *range_table;
+	ExprState  *qualexpr;
 
 	TransitionCaptureState *transition_capture;
 
@@ -800,6 +806,7 @@ DoCopy(ParseState *pstate, const CopyStmt *stmt,
 	Relation	rel;
 	Oid			relid;
 	RawStmt    *query = NULL;
+	Node    *whereClause = NULL;
 
 	/*
 	 * Disallow COPY to/from file or program except to users with the
@@ -853,6 +860,26 @@ DoCopy(ParseState *pstate, const CopyStmt *stmt,
 											NULL, false, false);
 		rte->requiredPerms = (is_from ? ACL_INSERT : ACL_SELECT);
 
+		if (stmt->whereClause)
+		{
+			/* add rte to column namespace  */
+			addRTEtoQuery(pstate, rte, false, true, true);
+
+			/* Transform the raw expression tree */
+			whereClause = transformExpr(pstate, stmt->whereClause, EXPR_KIND_COPY_WHERE);
+
+			/*  Make sure it yields a boolean result. */
+			whereClause = coerce_to_boolean(pstate, whereClause, "WHERE");
+
+			/* we have to fix its collations too */
+			assign_expr_collations(pstate, whereClause);
+
+			whereClause = eval_const_expressions(NULL, whereClause);
+
+			whereClause = (Node *) canonicalize_qual((Expr *) whereClause, false);
+			whereClause = (Node *) make_ands_implicit((Expr *) whereClause);
+		}
+
 		tupDesc = RelationGetDescr(rel);
 		attnums = CopyGetAttnums(tupDesc, rel, stmt->attlist);
 		foreach(cur, attnums)
@@ -1001,6 +1028,7 @@ DoCopy(ParseState *pstate, const CopyStmt *stmt,
 
 		cstate = BeginCopyFrom(pstate, rel, stmt->filename, stmt->is_program,
 							   NULL, stmt->attlist, stmt->options);
+		cstate->whereClause = whereClause;
 		*processed = CopyFrom(cstate);	/* copy from file to database */
 		EndCopyFrom(cstate);
 	}
@@ -2535,6 +2563,10 @@ CopyFrom(CopyState cstate)
 	if (cstate->rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
 		proute = ExecSetupPartitionTupleRouting(NULL, cstate->rel);
 
+	if (cstate->whereClause)
+		cstate->qualexpr = ExecInitQual(castNode(List, cstate->whereClause),
+												&mtstate->ps);
+
 	/*
 	 * It's more efficient to prepare a bunch of tuples for insertion, and
 	 * insert them in one heap_multi_insert() call, than call heap_insert()
@@ -2580,6 +2612,16 @@ CopyFrom(CopyState cstate)
 		 */
 		insertMethod = CIM_SINGLE;
 	}
+	else if (cstate->whereClause != NULL ||
+			 contain_volatile_functions(cstate->whereClause))
+	{
+		/*
+		 * Can't support multi-inserts if there are any volatile funcation
+		 * expressions in WHERE clause.  Similarly to the trigger case above,
+		 * such expressions may query the table we're inserting into.
+		 */
+		insertMethod = CIM_SINGLE;
+	}
 	else
 	{
 		/*
@@ -2683,6 +2725,13 @@ CopyFrom(CopyState cstate)
 		slot = myslot;
 		ExecStoreHeapTuple(tuple, slot, false);
 
+		if (cstate->whereClause)
+		{
+			econtext->ecxt_scantuple = myslot;
+			if (!ExecQual(cstate->qualexpr, econtext))
+				continue;
+		}
+
 		/* Determine the partition to heap_insert the tuple into */
 		if (proute)
 		{

src/backend/nodes/copyfuncs.c

@@ -3309,6 +3309,7 @@ _copyCopyStmt(const CopyStmt *from)
 	COPY_SCALAR_FIELD(is_program);
 	COPY_STRING_FIELD(filename);
 	COPY_NODE_FIELD(options);
+	COPY_NODE_FIELD(whereClause);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1222,6 +1222,7 @@ _equalCopyStmt(const CopyStmt *a, const CopyStmt *b)
 	COMPARE_SCALAR_FIELD(is_program);
 	COMPARE_STRING_FIELD(filename);
 	COMPARE_NODE_FIELD(options);
+	COMPARE_NODE_FIELD(whereClause);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -2962,7 +2962,8 @@ ClosePortalStmt:
  *****************************************************************************/
 
 CopyStmt:	COPY opt_binary qualified_name opt_column_list
-			copy_from opt_program copy_file_name copy_delimiter opt_with copy_options
+			copy_from opt_program copy_file_name copy_delimiter opt_with
+			copy_options where_clause
 				{
 					CopyStmt *n = makeNode(CopyStmt);
 					n->relation = $3;
@@ -2971,13 +2972,20 @@ CopyStmt:	COPY opt_binary qualified_name opt_column_list
 					n->is_from = $5;
 					n->is_program = $6;
 					n->filename = $7;
+					n->whereClause = $11;
 
 					if (n->is_program && n->filename == NULL)
 						ereport(ERROR,
 								(errcode(ERRCODE_SYNTAX_ERROR),
 								 errmsg("STDIN/STDOUT not allowed with PROGRAM"),
 								 parser_errposition(@8)));
 
+					if (!n->is_from && n->whereClause != NULL)
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("WHERE clause not allowed with COPY TO"),
+								 parser_errposition(@11)));
+
 					n->options = NIL;
 					/* Concatenate user-supplied flags */
 					if ($2)

src/backend/parser/parse_agg.c

@@ -523,6 +523,14 @@ check_agglevels_and_constraints(ParseState *pstate, Node *expr)
 
 			break;
 
+		case EXPR_KIND_COPY_WHERE:
+			if (isAgg)
+				err = _("aggregate functions are not allowed in COPY FROM WHERE conditions");
+			else
+				err = _("grouping operations are not allowed in COPY FROM WHERE conditions");
+
+			break;
+
 			/*
 			 * There is intentionally no default: case here, so that the
 			 * compiler will warn if we add a new ParseExprKind without
@@ -902,6 +910,9 @@ transformWindowFuncCall(ParseState *pstate, WindowFunc *wfunc,
 		case EXPR_KIND_CALL_ARGUMENT:
 			err = _("window functions are not allowed in CALL arguments");
 			break;
+		case EXPR_KIND_COPY_WHERE:
+			err = _("window functions are not allowed in COPY FROM WHERE conditions");
+			break;
 
 			/*
 			 * There is intentionally no default: case here, so that the

src/backend/parser/parse_expr.c

@@ -1849,6 +1849,9 @@ transformSubLink(ParseState *pstate, SubLink *sublink)
 		case EXPR_KIND_CALL_ARGUMENT:
 			err = _("cannot use subquery in CALL argument");
 			break;
+		case EXPR_KIND_COPY_WHERE:
+			err = _("cannot use subquery in COPY FROM WHERE condition");
+			break;
 
 			/*
 			 * There is intentionally no default: case here, so that the
@@ -3475,6 +3478,8 @@ ParseExprKindName(ParseExprKind exprKind)
 			return "PARTITION BY";
 		case EXPR_KIND_CALL_ARGUMENT:
 			return "CALL";
+		case EXPR_KIND_COPY_WHERE:
+			return "WHERE";
 
 			/*
 			 * There is intentionally no default: case here, so that the

src/backend/parser/parse_func.c

@@ -2370,6 +2370,9 @@ check_srf_call_placement(ParseState *pstate, Node *last_srf, int location)
 		case EXPR_KIND_CALL_ARGUMENT:
 			err = _("set-returning functions are not allowed in CALL arguments");
 			break;
+		case EXPR_KIND_COPY_WHERE:
+			err = _("set-returning functions are not allowed in COPY FROM WHERE conditions");
+			break;
 
 			/*
 			 * There is intentionally no default: case here, so that the

src/include/nodes/parsenodes.h

@@ -1969,6 +1969,7 @@ typedef struct CopyStmt
 	bool		is_program;		/* is 'filename' a program to popen? */
 	char	   *filename;		/* filename, or NULL for STDIN/STDOUT */
 	List	   *options;		/* List of DefElem nodes */
+	Node	   *whereClause;	/* WHERE condition (or NULL) */
 } CopyStmt;
 
 /* ----------------------

src/include/parser/parse_node.h

@@ -69,7 +69,8 @@ typedef enum ParseExprKind
 	EXPR_KIND_TRIGGER_WHEN,		/* WHEN condition in CREATE TRIGGER */
 	EXPR_KIND_POLICY,			/* USING or WITH CHECK expr in policy */
 	EXPR_KIND_PARTITION_EXPRESSION, /* PARTITION BY expression */
-	EXPR_KIND_CALL_ARGUMENT		/* procedure argument in CALL */
+	EXPR_KIND_CALL_ARGUMENT,		/* procedure argument in CALL */
+	EXPR_KIND_COPY_WHERE		/* WHERE condition in COPY FROM */
 } ParseExprKind;